Why develop knights before bishops? So, I've found the general advice that you should develop your knights before your bishops. 
Reasons I think are:
1) The knights almost always go to the main squares (c3,c6,f3,f6) (or sometimes to d7 the black knight in the queen's gambit) but, generally, you know in advance that you are likely to want your knights there. However, the bishops you don't want to commit to a square and "reveal" your intentions.

Of course this rule has a lot of exceptions, starting with the ruy lopez Bb5, italian game Bc4 are better for white than 3 Nc3.

But I've seen this rule for example here1. In that quora question, 5 answers reference my intuition that i describe below. Here also argues that while not universal, it is recommended normally. Other authority sources were founded using this article , in that , it can be found that Emmanuel Lasker and Reuben both recommend it:


  Bring out your knights before developing the bishops.


Intuition: is not to commit /reveal information by moving the bishop, and since the knight has almost only 1 great square in most openings (c3,f3,c6,f6) then there is nothing to hide and sooner or later it will be needed there, so just move right now. Keeps the bishop options open.
Commitment (versus flexibility) indeed is the main reason for this piece of advice. In particular: 


The first moves are (usually) all about fighting for control over the center. The knights are superior to the bishops in that regard because from their "natural" development squares, they have access to 2 of the central squares (e.g. a knight on f3 reaches both d4 and e5 - attacking and defending the center at the same time), while the bishops can control at most one (e.g. Be3 reaches d4 and Bf4 reaches e5, so the bishop can only attack or defend the center, but not both) unless they are fianchetto'ed, but that has the cost of two moves to develop instead of one. 
Half of the options to develop the bishops in one move (Bf4, Bg5, Bc4, Bb5) makes them vulnerable to the opponent's pawns (Be2, Bd2, Be3 and Bd3 are safe) which might cause you to lose tempo, while the developed knights are usually safe from pawn attacks after their first move.
a bishop may attack a developed knight on the bishop's first move (e.g. 2...Nc6 3. Bb5), a knight cannot attack a developed bishop on the knight's first move. This slightly increases the value of developing the bishops only after a few moves.
never forget that this is just a broad rule of thumb, and there actually is a perpendicular one: To develop the pieces of one wing (typically kingside) before those of the other to enable you to castle as soon as possible. You will very often see the bishop on f1 to be developed before the knight on b1 (e.g. in the Ruy Lopez or Giuoco Piano), contradicting the rule to develop knights before bishops. From the fact that these two openings are by far more popular than the Four Knights, you can tell that these moves are very strong regardless!