Why your relative position to the preflop aggressor (PFA) is important:
Imagine there are 8 bets in the pot and you sit directly to the right of the PFA. You have a flush draw. The flush draw gives you 9 outs which translates to an equity of around 35%. This is the chance to hit 9 outs from flop to river. This equity is almost independently from the number of players in the pot as your are drawing near to the nuts (in a lot of cases you draw to the nuts). The table may now look like this (H = Hero, A = Aggressor, o = other players)
HAoo
If you bet and the PFA raises, the other players will face two bets to call. Their pot odds are 11:2 = 5,5:1. That means they can't call many hands, mostly they will fold. What remains is you against the PFA who has almost always a made hand. You have an equity of 35% versus 65% of the aggressor and are an 2:1 underdog.
Now compare it with this:
Table structure HooA
If you bet the other players are getting pot odds of 9:1 or better. They will much more likely call your bet and stay in the pot. This is a very good thing as this improves your implied odds as there will be more players that can pay you off. If the PFA raises and you call the others will get odds better than 12:1 so no one will fold. You might even reraise, giving the others between 6,5:1 and 7:1 pot odds. This is still better than the 5,5:1 odds in the example before and because the others already called a bet they will be much more likely to call again. The other important effect is that your relative equity compared to others is excellent. As your equity is almost independent of the number of players staying in the pot you still have close to 35% equity. This means the other 65% equity are distributed among the remaing 2 to 3 players. With 4 players in the pot this means the average equity of the other players is only 65/3 = 22%. You are now not an underdog but a favorite to win the pot compared to the other players! this means you can raise for value. Even if only two other players remain, their average equity is 65/2 = 33% which is still worse than yours.
In general with a draw it is preferable to keep as many players in the pot. The more players remain in the pot the better your relative equity compared to the others is and the better your implied odds are.
There are two distinct exceptions: the first exception is the play in short handed pots. When there are 2 opponents or less in the pot you should play your draws aggressively. In this case you play for foldequity. You have a good chance that all remaining players fold because it is less likely that someone has hit the flop so their hands can't stand much action. If you play your draw aggressively you will win the hand a lot of time even without improving. This is of course even better than winning only when you hit your draw.
The other exception is in big pots on the flop where you might have other outs than your strong draw. These other outs would be pair outs. By playing aggressively you may get another player to fold who blocks your pair outs. For example he has the same high card as you but with a better kicker but no other draw as a backup. If he will fold you gain additional outs which improves your chances to win the pot. In big pots litttle improvements of your chance to win the pot can be worth a lot. For example if you gain 2 additional outs in a 15 bet size pot this is worth 9,1% * 15 = 1,36 bets. (This is derived from the probability to hit those 2 outs until the river.) The value of those two additional outs is worth more than the cost of the raise.
Preflop play: there are other good hands that you can hit on the flop. 2-pairs, trips will win you the hand a lot of the time. This combined with the flush draw makes playing the hand profitable if your odds are 9:1. Also, if your odds of hitting a good hand is better than the pot odds you get you should play the hand. You might even have implied odds on top. 9:1 pot odds is better than 8:1 odds for hitting the hand.