Its roughly 3% it adds in terms of winning a hand.  Which sounds small, but is a decent amount, definitely enough to where some suited hands are shovs while the same hand unsuited is folds(almsot all ranges have wider suited ranges for example).

A lot of the value in suited cards, isnt necessairly in its flush making capabilities, but its draw making capabilities.  FD and combo draws are extremely powerful, especially heads up and especially when shallow stacks against aggressive players.  You can play them fairly fast, with never being bad, and maximize FE.  So if someone is c betting a lot and you have a decent draw, you can effectively gain a lot of value by simply exploiting your FE.

People overvalue a lot(in MTTs at least), the value of making flushes.   Most of the value in making flushes come in cooler spots, which you are not really making much no long term, because you will almsot just as often find yourself in the reverse spot(set v flush, or two pair v flush, etc.).  The value comes from maxtop imizing value when you dont hit flushes, adn knowing how to minimize losses in spots where people will chase down or simply shove thinking , “Flush draw, got to get it in!”. 

Having a flush draw doesnt make getting it in ok, even if you know your flush is always good.  This is where value comes in, knowing how to ply your draws, when to go for FE, when call because you are getting direct odds.