As far as adding “str” to the end of a word, it’s just because it’s Nostr. There isn’t any deeper meaning, it’s the closest thing to a “trend” that exists here. 
Since there’s no algorithm to automatically sort and dispense information to you, we create our own categories and I suppose adding “str” was just something that someone started doing a couple years ago that people still do.
That’s not to say that, for example, you have to add it, as in #art , which is a perfectly valid hashtag. I hope I wasn’t confusing in that explanation .