I bet this because I want to build this small pot. A 3/4 bet is so small that you should get callers with even middle pair. Depending on your opponents, a pot sized bet will not protect your hand against the draw because the bet is going to be so small. They will take a shot at you for the implied odds of catching their draw at least until the turn.

This is a hand where I'm willing to string along the draws (while still building pot). I want to win money with my set, and if they hit their draw I haven't invested much and the pot is unlikely to get out of control. On top of that, I will still have outs.