Well, for me, the big thing that MSBT would show me was if I missed a casted spell. This would generally only happen if I had switched out a trinket or had forgotten to take off my pvp gear and put my pve gear on.

Other than that, it gives you a general idea of how hard your spells and abilities land for. As a mage, BIG NUMBERS are why I play, so that's the huge thing with MSBT. Outside of that, it's useful for tracking whether my numbers are dropping low or if they are going really high, like if I have a damage buff.

For a holy paladin, for example, if, let's say, your Holy Light normally lands for 10k healed (normal, not crit), and then suddenly you see it landing for 5k non-crit, you can see that and be like... why is my healing 1/2 effective. Is there a debuff on me? Is there a debuff on the tank?

Speaking of tanking, it allows you to see incoming damage too, which is extremely useful in pvp. Let's say there is a mage, hunter, and warlock all at range, and you start getting blasted by one of them. Your scrolling text can show the icons of the spell abilities that are hitting you, so you can identify right away who is attacking you.

Finally, there is also a feel to it. You just get a feel for the damage/healing you are doing, and the damage/healing you are taking.

That being said, it's not a completely necessary addon, but for me, I just like seeing what my spells and abilities are hitting for, and the default "over the mob's head" thing is really crappy... especially in AOE situations, where MSBT can consolidate your damage. Instead of seeing 10 numbers pop up, MSBT can consolidate those numbers by saying 125,837 damage (6 hits, 4 crits).

It makes it much cleaner in my opinion.