The Jewish settlements in the Sinai that Israel is preparing to turn over to Egypt include a showcase town that once was envisioned as the nucleus of a major seaport complex of 250,000 people and a village where Prime Minister Menachem Begin recently bought a retirement home.

Taken as a whole, however, the nearly 20 settlements Isreal has built in the Sinai Peninsula are the least important, both economically and strategically, of the more than 90 it has established in Arab territories captured in the 1967 war.

The settlements on the West Bank beside being closely identified with land that religious Jews consider rightfully theirs, form a picket line of defenses that shield Israel from Jordan to the east.

The Golan Heights settlements in addition to their thriving agriculture, form an obstacle course on Syria's likeliest invasion route into Israel and deny Syria use of the vast promontory as a base for shelling Israeli farms and villages below.

By comparison, the Sinai settlements, on land that Israelis have always considered someone else's, have only a marginal role in the Jewish state's economy and their security function had diminished in an age of electronic sensors and monitors.

Nearly all of the Jewish settlements in the Sinai, except for three along the Gulf of Aqaba, are clustered in the territory's northwest corner, where it meets the southern end of the Gaza Strip.

The purpose of this cluster of settlements was to block the Rafah Approaches - a narrow corridor between the Sinai desert and the Mediterranean that, throughout history, has been a frequent invasion route.

During recorded history, by Israeli count, 45 armies have passed through the Rafah Approaches - from the 18th Dynasty of the pharaohs to the troops of the late President Gamai Abdel Nasser.

Between 1948 and 1967 the area was the scene of some of the fiercest Egyptian-Israeli battles and Israel decided to put it firmly under its domination by colonizing it.

Under a plan drawn up by Moshe Dayan, Israel decided in 1972 to build a seaport and regional center of 250,000 people at Yamit, two miles from the southern end of the Gaza Strip.

But the October 1973, war interrupted work on Yamit and it also prompted Israel to take a second look at the project. Blocking tank corridors seemed more a job for satellites and missiles and Israel recognized that it might soon be returning the territory to Egypt.

Construction at Yamit continued, but at a vastly reduced scale. Nonetheless, with a population of about 3,000 and an investment of more than $25 million, Yamit is the biggest Israeli settlement in the occupied territories.

Near Yamit, at Neot Sinai, a small civilian settlement, Begin bought a modest house a year ago that, according to reports at the time, was to be his eventual retirement home.

Of the nearly 20 Jewish settlements in the Sinai, most are civilian and most of those are moshaving, individual leaseholds that are farmed collectively. Only a few are kibbutzim, the traditional Israeli collective settlements.

While many of the Sinai settlements began as paramilitary bases that combined agricultural pioneering with military service, only two or three apparently still have military status.

Israel has used the controversial pattern of establishing military outposts and later turning them into civilian settlements throughout its occupied territories.

Military outposts in occupied territory are permitted by international law. The United States and most other countries consider the Jewish civilian settlements illegal but Israel contends they are not.

Yitzhak Rabin, then Israel's prime minister, outlined in January 1977 the purpose of Israel's settlement policy in each of the occupied territories: To strengthen the confrontation lines along the Jordan River and the Golan Heights, to bolster Jerusalem and the Hebron hills, and to block the zone south of Gaza.

The placement of the settlements on the West Bank and Golan Heights - about two-thirds of them civilian and the rest military - reflected this policy until Begin's government began allowing settlements by ultranationalist groups at traditional Jewish religious sites.

Although Israeli officials, after the 1967 war, ambitiously spoke of plans for settling 2 million Jews in occupied Arab territories, reduced immigration into Israel and political considerations have kept the number much smaller.

Most of the 90-odd settlements have no more than a few dozen residents and, including Yamit, their total population is little more than 10,000.