Why play on draw?

why does many players bet when they actually is waiting to catch a draw???

Excellent question. The best reason for calling bets on a draw has to do with "pot odds." I other words, think like a casino and ask your self: what actions are going to net money over the long run.
Example
Let's say everyone has the same amount of money on the table. You have 6 7 in the big blind and after three people, from the other side of the table call, you suddenly notice there is a big wall mirror behind them. You can see all their cards. They're holding AA, KK, QQ, all unsuited. The flop Q, 4, 5 (rainbow ie all unsuited). Wow, you check trying to catch your straight for free and all three of them go all in. Assuming this isn't money essential for your survival, in which case you shouldn't even be there, what should you do? What are the pot odds? The money already in the pot is offering you 3 to 1 for your money. (If four other people were in the pot the odds would be 4 to 1.) Now, the odds of catching your straight are about 2 to 1. Over the long run you'll make money by calling this. Any casino would be delighted to cover those odds. In a ring game, assuming more money is in your wallet, you call. In a tournament, you have to decide if you're willing to risk the likelyhood of going broke (yes, you probably will lose.) over a strong shot at the big money in the final table IF you triple up.

GL,
Blimp

Odds of catching your open ended straight is 6-1. Not sure where you get the 2-1 from. You have 8 outs with 47 unknown cards remaining. You need a minimum of $600 in the pot to call a $100 bet.

The reason people bet on a draw is to

1) Possibly take down the Pot right there
2) Control the Pot odds
3) Semibluff

I think it is all about the semi-bluff today. Everyone wants to take in the huge pot and knock someone out. For most people that can only happen with a huge hand, betting on draws gives you the opportunity to score your huge hand or maybe win right there if the other players fold.