Because for whatever reason, even though they are mechanically balanced, some powers are probably going to be more appealing than others. Either due to their overall utility, satisfaction, or just how they appeal to a player.
So if you can pick let's say you can pick three powers from your class. You find two powers that appeal. But your options for that third, none of them look good to you.
With the feat, this lets you look at someone else's class list. And of those list, you can pick the most appealing to you off of that list.
That list is going to only get BIGGER, not only as you level, but as new books come out. You don't get to just look at YOUR first class's list of abilities, but your other class's too.
So you can pick the most appealing, satisfying, and useful abilities from two lists, not one.
To put it in 3e terms, let's say you're a 6th level cleric; you can cast say, 3 spells of 3rd level. But you only see two really good ones on the cleric list. Let's say that now you could pick a spell off the wizard's list, so you can pick the best 3rd level Arcane AND divine spells. All 3rd level spells are supposed to be equal with the other 3rd level spells, but we all know that some are better than others. Contagion doesn't hold a candle to searing light, etc.