ASSA is the same thing as .ass btw.
Unless you want to do something more than just basic text on a video, there's really no good reason to use .ass over subrip(.srt) which is likely the most supported subtitle format.
.ass just gives you a lot of control over how the text is displayed. You can change the font of individual lines, color them, move them to a specific position, scale them bigger or smaller, add gradients to the text, fade effects, stuff like that. You can even draw shapes if you want to block some text on screen and add subtitles over that part.
Here's an example of something fancy I did back in the day, the "B&L's Style Romance!" text in that picture is 100% soft sub, you can toggle it on and off. It's 9 different subtitle lines(
and possibly 2 more lines for the gradient and something else, forgot how that works 7 of which are to create the gradient effect, it only shows a slice of each line of different colors) stacked onto each other to mimic the style of the japanese title.
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Edit: One thing I forgot about that could be useful with .ass even if you only want basic text is that you can set an actor name for each line which doesn't get displayed into the subs so if you find it helpful to know who is saying what when editing, that's one thing .srt can't do.