What is ASS?

Advanced SubStation Alpha, the format serious subtitling actually uses. Fonts, colours, positioning, rotation, karaoke timing - anything you've seen a fansub do, it was doing it here. Converting to SRT throws all of that away and keeps the words.

What is WebVTT?

The subtitle format built for HTML5 video - what a browser loads into a track element. It's SRT with a header, a full stop instead of a comma in the timestamps, and optional positioning and styling that most people never touch.