The advantage is reusing existing themes. TextMate themes as a standard are the de facto not-great-but-used-everywhere thing and in this case it means you can style your blog or whatever with what you use in VSCode or Sublime or whatever.
TextMate 1.x was the king of Mac text editors back in its day. Is anyone actually still using TextMate today?
I don’t use TextMate anymore but I surely would have stuck with it longer if it had a debugger and language server. I panned all of the non-native editors that came between, but sadly had to admit that VSCode fit my workflow a lot better.