Poker If you're not getting the right odds to call, why do people raise?

From what i've seen from a lot of people, they say that if they don't have the odds they need to make a call then they'll re-raise etc; Why is this case? As you're still putting more money in than you might get back right?

Like let's say you have a flush draw with a 30% chance of winning it, and you're opponent is offering 2/1 odds. Of course calling here wouldn't be right unless you have good implied odds correct? So why do people say that instead of making the call that you'd re-raise their initial bet? As you're still putting money in the pot aren't you?

I hope someone can understand this. Thanks

Source.

 

Hope this helps. Flush draw on turn is 4-1 against. If no implied odds in order the pot must pay you 4-1 to break even.
You state the pot was only offering 2-1?
Never make a profit on that matchup.
You mention "..they.." raise. Math states that not profitable unless they have a plan on latter streets to try & buy the pot. Pure pot math calling a 4-1 drW with 2-1 return is. -Neg expectation.