If you aren't going to raise at all if you're playing a low limit holdem game, you aren't gonna get very far with your chips.

If you just call someone's bet, you aren't gonna know where you stand.

Does his bet tell you he has high pair? mid pair, a strong draw?

Sometimes if I have mid pair with a big kicker and someone bets twice the big blind, I'll reraise him just to see what he'll do. Maybe he doesn't really have the top pair and was just betting for value. You won't really know if you just call him. Because the next time around he's gonna raise even bigger and your little call isn't going to tell you one thing.

Now assuming he isn't slowplaying and you reraise and he just calls, it could mean he has top pair with a so-so kicker, or it could mean he has mid pair and doesn't believe you have top pair either. If he folds, then you know he didn't have top pair and didn't want to duke it out with you.

Just calling anyone's bet is inviting them to be the aggressor against you on the next round of betting. You've got to raise to establish whether they will call you back, reraise you, fold, or go over the top. It gives you a better indication of what they have. This will help you plan your next move/bet on the next turn.