Just a way to make it show, mainly because people like to skim through posts and it can help people catch what's actively happening at least.
I personally just used it to be eye pleasing though, once in a while my eyes would hurt just going through text that was all the exact same. The bold gave something a bit different that sort of stopped the blur/annoyance going on with them gave my eyes a chance to reset. I wouldn't have much (if any) need of such a thing today though.
The different colours case is only done by people for other characters (and I ever did it, it was a test run and I scrapped it quickly), but I find it's normally only used when people generally don't bother details who says what. So instead of say:
Ryan ran down the hill and said "I'm going to the supermarket", and Hordor replied "Hordor".
It would be:
-Using Bold and Italicize because coloured text is sadly not implemented on this Guild yet. :(
Ryan ran down the hill and said
"I'm going to the supermarket",
"Hordor".
Brovo said
As a GM I find most don't actually read the full post whether they should or shouldn't. So I bold the dialogue to make them think that is all the info they need.And then they die by chainsword.