"So you're going to South America," said my friend. "Why?"

Funny thing about South America. No one asks "Why?" when you say you're off to Europe or even to Africa. Yet it crops up all over the place when you mention South America. After a recent 2 1/2 month trip around the continent, though, I think I know why the why. Furthermore, I finally have an answer.

Image, I suspect, is at the bottom of the question. Apparently it is widely believed that South America is largely a vast, wet, green jungle oozing boa constrictors (the exceptions are the Argentine pampas, where the gauchos play, and Rio de Janeiro, which is the last of the really "hot dog!" sexy cities). Moreover, this line of thinking goes, South America is populated largely by short, fat men who run around in military uniforms wearing pinky rings and pistols, and slender but astonishingly endowed women who wear Band-Aids as bathing suits.

It is also well understood here that South Americans are much given to revolution, thereby making tourism risky - but never mind, if you drink the water you won't live anyhow. So much for the view form North America.

Latinos return the compliment, partly by their certainty that Al Capone (or a reasonable facsimile) is alive, well and operating in Chicago, Miami, Detroit, Washington and New York, to mention just a fes spots. Nor is it any secret from them that any U.S. citizen on the road is either a fullfledged spy or intimately related to one. At the same time, Americans are pathetically naive, if not certifiably simple-minded, and will buy absolutely anything and pay through the nose.

As far as I can tell, our ignorance of them is more profound than theirs of us. And in a sense, that's the answer to, "Why go?" Because we don't know it, South America is still a big, often beautiful, highly enlightening adventure.

The bonus is that, except for Brazil and Venezuela, it's also pretty much within the means of low-budget travelers once you've saved up enough for the air fare. Even Brazil and Venezuela fall into line financially if you're adventurous and willing to make some compromises on comfort. (Cheapest are Argentina, Colombia, Ecuador and Uruguay.)

A South American adventure can start before you ever leave the United States.Finding answers to your where-to-go questions requires the skill of a Serpico. In my experience, Latin American tourist information outlets in the United States are good for a lot of adjectives and little else. U.S. guidebooks aren't much more helpful.

The British, however, give a hand in the form of "The South American Handbook." Few bookstores seem to have cottoned to the fact that this is theessential work on South America, but if all else fails it can be ordered by mail through Rand McNally, 10 E. 53d St., New York, N.Y. 10022 ($16.95 plus $1.25 postage for the 1976 edition).

Planning, of course, is the key. The people who stand to get most from a South American trip - and the most for their money - are the ones who know what to expect.

"It's a continent with three faces," explained a Uruguayan teacher I met. "There's the African side all along the east coast from the Guianas to the state of Bahia in Brazil. Then thre's the European part from southwestern Brazil on into Chile. And in the north and west you see the Andean-Indian."

The mix is such, however, that you can take a safari in "Europe" (Paraguay has its own Piladeophia, which is at once the center of a Mennonite Community and the takeoff point for jaguar and puma stalking trips in the Chaco jungle) or spend your "African" time looking at 18th-century European architecture in Bahia.

You can also stay at Hollywood-on-the-Amazon, an incredibly well-manicured mirage of a resort that Varig Airline has just opened on the bank of the river outside Manaus, Brazil, or sip superb German-style beer served by descendants of the Incas in a La Paz beerstube.

But even though South American has the regulation casinos, nightclubs and posh resorts, they're not around in quantity, nor are they the "main dishes." With few exceptions, the grand hotels are not all the grand, and the lesser ones - especially in small towns -can be like '40s motor courts, or worse.

And there are political flareups and tensions from time to time. Arriving in Lima, I ran straight into an 8:30 p.m. curfew (it's now 1 a.m.) and a shift in currency exchange rates that, had I bought Peruvian soles before arriving, would have had me losing a bundle. Some South American currency rates are tricky to learn about ahead of time, the water is suspect in most parts of the continent, and you do need to check with your doctor about local diseases and precautions if necessary.

In sum, South American countries are not the best bet for the city-oriented luxury and entertainment seeker, and Mexico can surpass them by virtue of proximity and similar color. But they are pretty close to heaven for the nature-and-old-ruins lover, whitewater river rafters, fishing fans, campers, hikers, unfussy adventurers and the culturally curious. If you like the offbeat, you can almost bank on being dealt the equivalent of a royal flush.

In Belem, Brazil, for instance, there's a zoo of sorts where I watched a barefoot man using his feet to prod crocodiles out of his way so that he could clean their pond. Strangely enough, he still had all his toes. In the Venezuelan jungle, I met a red-haired Oklahoma prospector who casually undid a leather pouch and rolled his latest "haul" of rough diamonds across the table for me to examine.

In a Colobian river town, I sat with three other travelers drinking rum in a dirt-floored bar and wondering, vaguely, why a live pig was tied to the table leg. This trip I turned the corner on a cobblestone street in the Peruvian mountain town of Cuzco and found myself face-to-face with a living, breathing llama.

Perhaps the biggest thing to be aware of is bigness. You can't adequately see South America as many people "see" Europe, all in one gulp. The distances are vastly greater. Brazil alone is almost as large as the United States, and it takes 19 hours just to go from Brasilia to Rio by bus. Even traveling all by air, one South American country can fully occupy anyone with only three weeks or less. And you'd still need the fingers of both hands to count what you'd missed.

The beauty part of many people, though, is that when you go to South America, you go away and you get away.For the moment, anyhow, you can go a long, long while without spotting either a U.S. style chain motel or a Southern colonel's chicken shop, much less one of what's-his name's hamburger palaces.