It's just a trend to put z or s after your name because it sounds cool. idk if you get the idea, but for Thais, it's really cool. lol
about 10 years ago there's a trend to put 'zaa' after your name, it literally means 'cool' like if you're John, 'Johnzaa' means 'cool John' or 'John, cool guy'
these days 'zaa' is kinda old-man things, (lol) so they just make it z instead.
but if your name with the z is already stolen by someone else, (many online games can't use the name that someone already use it), you probably just change the Z to S because it sounds similar.
hope this answer your question. it's kinda cultural and hard to explain.