betting draws?

i hear a lot about people betting draws like for ex. when someone flops a 4-flush. could someone explain the reasoning behind this?

I'm just wondering because I've seen hand histories (no specific ones that I can remember unfortunately) where people generally agreed that betting a draw like this was correct despite the pot odds not being there.

When i'm drawing to a hand i will bet it out most of the time. This accomplishes two things. Firstly your opponent can fold, giving you the pot right there, and second it disguises your hand. Check calling down on a two flush board and then betting when the flush card hits is quite obvious and you wont get paid off much.

Bmxicle has pointed out two advantages of bettting with a draw.
The 3rd example is value betting, which can be done in specific situations.
Lets say you have a flush draw with 3 more people in the pot, which are all calling stations. You have 35% of completing the flush, and you get 4x the money if you hit, so it is correct to bet and increase the pot.

yeah forgot about that cause its more of a limit idea.

expanding on value betting, it also builds the pot for when you actually hit your hand. Assuming your opps will fold to a scare card, would you rather win just the preflop bets or a little extra on the flop?