not exposing the actual IDs for resources in your application
Slugs add a small additional complexity since you are not generating from the auto-incrementing ID on the database - your application must generate a candidate slug, check that it is unique on the database table, and repeat if necessary.
Like @tykus says, somethimes it might be an security issue with using the id, in those cases a uuid migth be a better option since they are not as easy to guess. However with the right middleware and authorizations a user should not be able to view things they don't have access to by just changing the id.