Many chess engines are (also) made for humans to play against.
Humans wouldn't want their opponent to play the exact same way every time. For a simple example, playing white, one wouldn't want the computer to always open 1. h4.
So, either the computer has to pick an opening, or the human opponent has to pick one.
Of course, that extends past the first move, so, assuming the computer to play deterministically, one would end up with the human opponent having to specify "let's play opening X, variant Y, as typically played by Z up to black's 15th move". It probably is easier to use a "set up a custom board" feature than to specify that.
And it wouldn't even stop there. The human specifying that particular opening likely wants to study that opening, not just the single continuation that the computer thinks to be the best one.
Also, as another poster said, a chess engine may use Monte Carlo simulation to pick a move, and that means using a random number generator.
Humans wouldn't want their opponent to play the exact same way every time. For a simple example, playing white, one wouldn't want the computer to always open 1. h4.
So, either the computer has to pick an opening, or the human opponent has to pick one.
Of course, that extends past the first move, so, assuming the computer to play deterministically, one would end up with the human opponent having to specify "let's play opening X, variant Y, as typically played by Z up to black's 15th move". It probably is easier to use a "set up a custom board" feature than to specify that.
And it wouldn't even stop there. The human specifying that particular opening likely wants to study that opening, not just the single continuation that the computer thinks to be the best one.
Also, as another poster said, a chess engine may use Monte Carlo simulation to pick a move, and that means using a random number generator.