I've been running some engine vs engine games out of boredom and noticed Stockfish, when playing as black, 90% of the time played c7-c5 on white's e4. I've never actually seen a human play this, in the few years of my playing. Is it a move that mostly professionals play? Is there a concrete reason for that move?
1...c5 initiates the Sicilian Defense. It is absolutely a respectable opening, and I am surprised you've never seen it before. I'll just quote Wikipedia:
The Sicilian is the most popular and best-scoring response to White's first move 1.e4. Opening 1.d4 is a statistically more successful opening for White because of the high success rate of the Sicilian defence against 1.e4. New In Chess stated in its 2000 Yearbook that of the games in its database, White scored 56.1% in 296,200 games beginning 1.d4, but 54.1% in 349,855 games beginning 1.e4, mainly because the Sicilian held White to a 52.3% score in 145,996 games.
Because the Sicilian is so good, it shouldn't be surprising that Stockfish plays the opening.
Any answer to the question will be as dangerously biased as the database behind it, and moreover Stockfish will have other opening "reasons" than a human (I play 100% c5, and that even without knowing theory...), but here are some values from the database behind the Lichess analysis board, as number of games and white win-white loss:
- c5 - Sicilian - 480000 - 6%
- e5 - Open - 230000 - 10%
- e6 - French - 130000 - 11%
- c6 - Caro-Kann - 79000 - 8%
- d6 - Modern - 40000 - 12%
- g6 - probably KID - 30000 - 7%
- d5 - Scandinavian - 22000 - 14%
- Nf6 - Aljechin - 20000 - 11%
Observe how good g6 scores, I guess because it mostly ends as Kings Indian and a no holds barred fight will ensue.
1...c5 is a great move that fights for the center even better perhaps than 1...e5. By playing ...c5, White will only be able to play d4 either by trading one of his central pawns for a side pawn or by preparing it with c3 (a move the b1 knight won't be happy about).
On the other hand, 1...c5 is not a developping move (and does not help development at all), so White can try to profit from an early lead in piece activity. The most ambitious plays to try to exploit that often lead to chaotic games with wild attacks.
Overall, the Sicilian Defense (1.e4 c5) is the most common response to 1.e4 in tournament play. Even if opening popularity is a matter of fashion, the Sicilian has stayed on top for a long time.