Many reasons. There is probably not a straight forward answer if you never tried to answer (its my first time) this question. But I’ll give it a go.

Most importantly and in the long run (and I’m talking about hundreds of years) future generations can study old games, to learn how the past was working. For the short term (I"m speaking about now for next decades) its partly about nostalgia so you can play the games, if companies fail to preserve them. Also preservation allows us to play and see the games in their initial state in example. Game developers also can study old games, which is important to make new ones. Reading books how the golden era was this and that is one thing, then playing those games and seeing how it works is another. Even if its not 100% authentic recreation, its still helps.

Why do you preserve old books, films, music, art? Why do we preserve old technical devices we found, old bones of animals or even humans? Compared to those, its much more complicated to preserve videogames, not only the bits and bytes as they are available, but also to have them playable. Videogames is part of our society and preserving them is preserving part of our humanity.