Here's why that matters: Yanukovych was democratically elected. Ukraine is what political scientists call a "transitional" government: It's not a fully entrenched democracy, and it's not autocratic, but it has features of both. The real danger with transitional governments is that they can revert to autocracy and authoritarianism pretty easily. That was a real risk in Ukraine in particular, especially after Yanukovych pushed through draconian "anti-protest" laws that limited free speech, assembly and media rights. Parliament pulled most of that law back, and now they're directly undercutting Yanukovych's institutional authority to crack down on protesters. And they're doing it though democratic processes.
That might not sound like it would matter so much, but it really does. It symbolically places Yanukovych, especially if he ignored parliament on this, outside of both the democratic processes and outside rule of law. He's stuck now – if he defies the resolution, he'll be basically labeling himself as anti-democratic, which will really badly weaken him institutionally.
This also goes a long way toward reducing the odds that Ukraine's military would intervene on Yanukovych's behalf, which was a serious concern. It reduces the odds of an outside Russian intervention as well, given how difficult it would be to maintain the fiction of intervening the protect Ukrainian democracy from "insurgents." (One caveat: It's not clear whether there were enough legislators present to meet a quorum, although some Ukraine-watchers say it was; 236 out of 442 seats.)
And it even delegitimizes Yanukovych within his own party. Of the lawmakers who voted for the resolution undercutting him, 34 were from his own party. So it will be tougher for him to claim popular legitimacy even from his own political movement.
There was a time, early on in this three-month crisis, when Yanukovych could legitimately claim to have a popular mandate from his 2010 election and from the significant number of Ukrainians who supported his policies, including in rejecting the trade deal with the European Union, which had only about 42 percent support. But that's been getting tougher and tougher for him, and his own parliament just made it pretty close to official.