Hello One of the biggest differences between tags and categories is that all WordPress posts must be filed under a category, but they don't have to have tags. If you don't give your post a category, WordPress will automatically assign it to the default category.
Well tags are those keywords which are chosen by you according the post, which are keywords when anyone search in Google or any search engine, these keywords then placing the respective website according its ranking in search result. I would recommend to have long trailing keywords to be used in your tags of particular individual post of website which you own. You can also Google "how to use tags in WordPress" and it will help you lot in placing keyword rich tags in your posts. Thanks in advance!
Why are you posting things that will never have a related post? I run a site that I built tags into, it's 15 years old so the tags are really important for linking old posts to new.