What's up with the micro bets?

[fourgapper]

Playing online 0.5/1 PL/NL Hold'em at PP, I encounter a lot of SB size betting on the flop and turn. What is the point of this, isn't it essentially a free card? If the bettor has a strong hand and wants to build the pot, isn't this a mistake if I have a strong draw, for example.

I see this betting pattern all the time, but am a beginner and there do not know if I'm missing a sublte aspect of the game here.

Many thanks.

4gapper