One main point is that the mod author gains some control over careless spoiler posts on their mods, protecting others who may not wish to read spoilers before they've played through the mod. If it was only controllable by the individuals reading the comments, then people would need to first read the post to identify it's a spoiler, by which point it's too late. That would also require saving each individual user's preferences on a per user, per thread basis, which could bloat the stored data for each user. And you'd need to be saving these preferences otherwise the setting would be worthless, as the next time that person viewed the same comment thread it would all be undone again.