I'll use Ne2 for the sake of clarity.
A few situations that immediately come to mind are
1) supporting d4 while making the pin Bg4 impossible (you can play f3)
2) The center is locked, meaning play will develop on the flanks, meaning control of the center with minor pieces is not very important, plus flank breaks (pawn to f4 or pawn to f3 to defend against black's pawn break) gain in importance. So Ne2 gets ready to move it to the g or f files, plus doesn't block the f pawn.
Another reason is that you might want to protect d4 with your knight, while not blocking in a fianchetto bishop on g2.
Or in some openings, like the Tarrasch variation of the French, white will play c3 to support the d4 pawn, and also Nd2, intending to move the queen's knight to f3, while the king's knight goes to e2. And then if a trade of pawns does take place, the e2 knight is the one that ends up on c3 later.
Yeah, those are good reasons too.
Some lower level players don't realize that a knight on f3 (for example) is not ideal when they've finachettoed their bishop on g2.
Sure it's not an uncommon occurrence, but in such cases white is always looking to reposition the knight, h4, d2, and such.
^