Discussion pages are an integral part of a wiki; they are a place for users to decide which direction an article should go in, what to include and not to include, and to place any other kinds of comments. User talk pages are also important, being a place where users can communicate about a wide variety of things, as well as receiving warnings from other users regarding unhelpful edits, and messages from Administrators about blocks. This extends to Talk Pages as well.
Talk pages and comment sections of a content page are restricted to discussions regarding how to improve the article; Random declarations and fandom do not belong here. Rather, they can be expressed on a User's talk page, a forum, or blog post.
These comments may end up outdated and seem useless; however, there are many reasons to keep them.
There may seem to be no purpose, but keeping discussions can be very useful. For example:
Keeping old discussions may prevent the same debate from happening repeatedly. For example, if two different sources provide differing information, leaving the discussion may prevent it being brought up repeatedly.
Providing a record of a user's actions and discussions with other users. Although it is possible to view the page history, it is much easier to look over a discussion.
Obviously, some discussion may not necessarily be an integral part of the article or user's talk page. For example, "OMG YOU GOT OWNED NOOB" or "ZOHMIGOSH LAWLS!!!!!" should be deleted. Spam, or an utterly pointless message, however, is the only thing which can be deleted, as it is not actually a discussion of any kind.