Until recently, only one reply per user per level was permitted. That was recently changed to where a second, third, etc., reply can be made but the cost for each increases an order of magnitude each time. So that's one reason you will see people reply to their own comment.

My aim when sharing content from a post is to add value (e.g., a cliff's notes of the article, a timeline from a podcast, links to Mastodon, etc.). Unfortunately, this content added can be verbose. A reply that is 30 lines long is like a wall of text -- the brain sees that and says, I'm not reading all that, I'll just scroll down. Whereas when there are short sections, a person is more likely to read the first (shorter) section.

This also lets a comment reply to my comment (from another user) be located just under the content being commented rather than below a long section.

So that's the intent -- to put the part with the most value to the reader in the first reply, then if there is more, put that in a second reply, and if needed a third and, possibly a fourth reply as well. By the way, it takes just one click to collapse a reply, which hides that reply and everything underneath it. And SN will remember that so if you come back to a post later, that reply will remain collapsed.

There's also another reason a person will reply to themselves, and that is because after the 10 minute edit window has passed, a reply is the only way to add a further thought, or to make a correction, or add an update, etc.