Fallout didn't *need* Fallout 2.
Half-Life didn't *need* Half-Life 2
Final Fantasy didn't *need* Final Fantasy 2.
Good games don't need sequels. Sequels aren't the same game, they're your chance to see the same basic mechanics but more awesome, and with more features that maybe they couldn't have put in the first time around, that's the basic idea behind it. Sometimes you just love something so much you don't want it to stop, but you can only play through the same level so many times. Sequels tend to get more funding as well.
Nowadays admittedly there is a trend towards epic trilogies of games and such (Assassin's parkour, Mass Effect), but even these have changes and try and build on what came before. They try to take the good from the previous, and discard the bad. Assassin's parkour 2 tried to make the game less repetitive than the original, Mass Effect 2 was inspired more by later Sci-Fi, which was much more darker and edgier (a point loads of people miss when they compare the two -_-).
Portal was a quirky, fun 3-and-a-bit hour gravity-and-portal puzzle-fest. So long as they keep the sequel using that same basic mechanic but maybe with a few new, interesting features, and of course new levels and so-forth, it'll be awesome.