What is the point of delaying the exchange in the Rossolimo? [fen &quot;&quot;]

 1. e4 c5 2. Nf3 Nc6 3. Bb5 g6 4. O-O (4. Bxc6) Bg7 5. Re1

Is there any point in maintaining the tension? Does it force any adjustments from Black? Is the timing irrelevant or is White really waiting for something?
The underlying reasons for delaying BxN are -

Preserving options
Saving tempi

Suppose in the final position black plays a6 forcing white to make a decision. Bf1 is a perfectly good alternative to BxN. It is one which will lead to an entirely different game, one which white may prefer.
In any case it makes some sense to wait for black to commit with a6 because then BxN comes without the loss of a move that playing it unprovoked would involve and it has provoked a weakening of the b6 square.
Again looking at the final position above, an immediate BxN leaves black with a choice of recaptures, either bxc6 or dxc6. White may not like the position after dxc6, in which case it makes sense to wait until black plays d6 first before exchanging. Alternatively white might not like the possibility of BxN bxc6 followed by d7-d5. Again if white waits for d6 then a tempo has been saved.
Keeping options open and saving time are features of the way stronger players play. Prematurely closing down options and squandering tempi are what weaker players do.