What is the reason (if any) for 1.e4 c5 2.Nf3 d6 3.Nc3? As title above, what is the point of playing 3.Nc3 (before d2-d4) after 1.e4 c5 2.Nf3 d6? Compared to the “normal” 1.e4 c5 2.Nf3 d6 3.d4?

[fen ""]

[Variant "Standard"]
[ECO "B50"]
[Opening "Sicilian Defense"]

1. e4 c5 2. Nf3 d6 3. Nc3 { Less common move } (3. d4 { Usual move })
It basically just stalls for a move before 100% committing to the open Sicilian. For example, suppose you were playing White and hated facing this Sicilian (1.e4 c5 2. Nf3 d6 3.d4 cd 4.Nxd4 Nc6). You could use the move order you described and after 3.Nc3, only play 4.d4 if your opponent doesn't play 3...Nc6. If they do play 3...Nc6, then go for some Closed Sicilian.

Also in online bullet some people have done the 3.Nc3 move order against me because it takes half a second to realize what's happening and then play 3...Nf6 instead of 3...cd.