White's options are kept flexible as White could intend to play the Réti Opening or the King's Indian Attack, although it can easily transpose into something else, including the Sicilian Defence. Other reasons for playing 1. Nf3 go something along the lines of:
White hates playing against Variation X, but doesn't mind Variation Y.
White plays 1. Nf3 as a request for information. If Black attempts to transpose into Variation X, which White hates, White will refuse to allow it and play an independent line instead.
If Black attempts to transpose into Variation Y, White will go along with that.
As an example, a player who hates facing the Queen's Gambit Declined but doesn't mind the Dutch Defence might play 1. Nf3. If it is answered with 1... f5 they will play 2. d4 transposing into the Dutch, but if 1... d5 appears then they will go for a King's Indian Attack with 2. g3. A player whose preferences were the other way around might answer 1... d5 with 2. d4 (allowing a QGD) but 1... f5 with 2. e4!?, the Lisitsin Gambit (not allowing the Dutch.)
One general point though: Because 1. Nf3 prevents 1... e5, it has more in common with 1. d4 openings (and some 1. c4 openings) than with those following 1. e4. It is also a popular way for White to set up a King's Indian formation.
Black can play almost anything comfortably at this point, and the following options are merely some of the more common replies.
Game | Result | Year |
Aronian, L. (2826) vs Carlsen, M. (2872) | 1-0 | 2014 |
Carlsen, M. (2876) vs Nakamura, Hi (2802) | 1-0 | 2015 |
Carlsen, M. (2864) vs Ding Liren (2806) | 1/2-1/2 | 2022 |
Carlsen, M. (2864) vs Ding Liren (2806) | 1/2-1/2 | 2022 |
Caruana, F. (2827) vs Carlsen, M. (2840) | 1/2-1/2 | 2017 |
Kramnik, Vladimir (2812) vs Carlsen, M. (2855) | 0-1 | 2016 |
Kramnik, Vladimir (2812) vs Carlsen, M. (2855) | 0-1 | 2016 |
Topalov, Veselin (2793) vs Carlsen, Magnus (2868) | 0-1 | 2013 |
Caruana, F. (2804) vs Carlsen, M. (2855) | 0-1 | 2016 |
Carlsen, M. (2876) vs Grischuk, A. (2781) | 1-0 | 2015 |