Kind of like how you can do [in-line links](a website) to link people to a website, allow the user to use the same syntax to create contextual information that appears when the user mouses over. This way users who know the context won’t have to slog through a massive wall of text while those who don’t can optionally bring themselves up to speed.
The problem with that is that you can’t
have spoilers within the line itself
As each spoiler requires a line break
and it just looks awful, both in the markdown, and especially when rendered as a comment.
Compare the almost seamless implementation in my example to the raw output of this comment.