There generally isn't a reason to use tags in questions and answers on any main Q&A site. It may happen in comments but it doesn't format there, anyway.
Tag syntax was originally designed for usage on Meta sites - we have separate syntax for normal and meta tags because both of them will be discussed within the scope of that Meta sites.
We still allow tag syntax on main sites primarily because of tag wikis. You can use tag syntax to link a variety of tags in a tag wiki description, from similar and related tags to tags that often get confused for each other.
Tag wikis use the same post formatting as questions and answers (they are post type 3 to the others being 1 and 2), so rather than split up how formatting is done for 1/X formatting types on 2/3 of post types on 1/2 of sites (since the meta sites still need them), we just consistently apply the formatting on all post types.