Not all comments are. I have written probably upwards of 100 top level comments in /r/askhistorians and I believe I've had one deleted over a period of 5 years or so. However, a lot of comments are written without checking the rules or consulting the FAQ. In fact, had you consulted the FAQ before asking this question you'd have found a whole subsection under the META-department that is devoted to why so many comments are deleted. If you want to read it, here it is.

If you want a more specific example, here is a great rundown of deleted comments by the master deleter himself, /u/Georgy_K_Zhukov.

As to purspose, that too is something you would have found had you bothered to look before asking. It's literally the first sentence in the subreddit rules:

/r/AskHistorians aims to provide serious, academic-level answers to questions about history. We have written these rules to support this aim and maintain the high standard of discussion this subreddit has become known for.