Board Thread:Support Requests - Getting Technical/@comment-27358554-20170310185430/@comment-27345308-20170311160029

It gathers the wikitext and passed parameters and renders it on your wiki instead of getting the rendered template from Community Central. It also takes a bit more to load so it would be better if you don't use it often, or at least not often on a single page (read: don't use it for your signature). That way, you can also pass parameters into it.

As for your second question, it's because when it gets rendered it also gets cached, and it takes a while for that cache to refresh, I suppose. I tried purging my global templates earlier too, it didn't really work so I had to wait. I think  refreshes them faster, though.