There are lots of reasons for for wanting to delete comments without wanting to delete any replies.
For example in this comment, I bring up the use of macros as a solution to fix a mouse issue. If someone brought up a point that this software was against the games TOS, I would want my comment removed as fast as possible because I view my comment as potentially harmful (and from context, it would have been obvious what the deleted comment would have contained). If there were other replies giving advice on how to solder (one of the other solutions I give), I would want those replies to remain visible to anyone else viewing the post. Some people, like me, might not have been aware that deleting a comment would remove all other replies, I’ve never seen a warning message that this would happen when deleting something.
Deleting the comment on mobile instead of editing out bad information is also a lot more likely because it’s a “one button fix”, and a lot more convenient than trying to edit the post (some apps don’t render markdown correctly, and may not display strikethrough text). With the above example, quicky deleting my comment would have been an appropriate way to remove harmful advice, and a reply warning about the use of macros and advice on how to solder would still be helpful without the context of the original comment they were replying to.