Short answer is that they exist so college students can read a book with the benefit of foresight and someone else's analysis and immediately jump to a level of analysis that would otherwise not be accessible until a second or third or more rereading.

Personally I think this is a terrible way to experience literature as it largely removes a reader's ability to analyze whether the novel is successful at maintaining tension and the element of surprise, while also biasing the reader toward certain interpretations. But some people don't have the time or are just intellectually lazy and want the easy way out.

(They also unfortunately perpetuate the idea that classic literature can't be spoiled because they're not meant to be entertaining, they're meant to be studied, and anyone who is looking for entertainment is just not intellectual enough for classics....but let's leave that for another day.)