Question with the book "Understanding Chess Move by Move"

I'm reading a book which is titled "UNDERSTANDING CHESS MOVE BY MOVE" by John Nunn, and I have a question in it.

In the game 3, the game begins with
1 e4 g6
2 d4 Bg7

and then, white plays 3 Nc3 and the author comments that
"White must choose whether to occupy c3 with his c-pawn or his knight."

If I were white I would play Nf3 to reinforce d4 pawn.
I guess this is also acceptable move
but why does the author recommend to put some material on c3 in first ?

I'd like to know why it is.
Thank you.

Thank you for your advice.
So, is it like this?

1, I'm thinking that the kingside attack will be effective.
2, so maybe f2-f4 can be needed.
3, on the other hand it is not likely to happen c2-c4 immidiately.
4, so before f-file, let's decide how to play on c-file!

Anyway, thank you very much for advising me !

Everyone, really thank you for your answers.
They helped my comprehension of openings.
Thank you very much. I have completely understood!

With the opening black chose, white has a lot of options. He certainly could play f4, but this wastes a bit of time and creates some weaknesses which much later on might be annoying, so it's more aggressive but it gives black a bit more counterplay as well.

There really aren't many bad setups for white. A Nf3 Nc3 setup is also fine. White just develops. Black will obviously strike back in the center, but white understands that his e4 and d4 pawns don't have to stay there forever, and because of his good development he would like the game to be open anyway. It's a modest setup, more so using the pawns as a cushion to develop comfortably than as a pawn roller.

>>Elubas
Thank you for your answer:D
Aha, I need more flexible way of thinking!

Yes, exactly. Both white and black in the position you give have many different possible plans and counters to them.

Thank you !
I want to become a high-rating player such as you...
I'll study hard !