This is a guess on my part, but sending users a PM to let them know why their post was deleted could help in some situations. For example, if someone replies to a documentation type of topic to say that there is an error in the documentation, our standard practice on Meta is to fix the OP and then delete the reply that pointed out the error. If the user was sent a PM letting them know that their suggestion has been incorporated into the OP, they are unlikely to feel a sense of loss about the post. If there was some way to publicly acknowledge that the deleted post’s author is now a contributor to the OP, that would be even better.
Other than posts that go against the community guidelines, the main reason I’m aware of for deleting posts is to improve the topic’s readability. There can be a real loss of information when posts are deleted though. I’ve wondered in the past if instead of deleting posts, posts that are seen as being extraneous to the topic could be hidden behind a button and still be available as search results. On a long topic with a lot of deleted posts, this could create a messy user interface, so I’m not sure that this is something that could be made to work.