More tables mean more hands, more spots, and potentially more profit over time. It can also stop players from getting bored or overthinking every decision.
The downside is obvious: if you add too many tables, your focus drops and mistakes go up. Multitabling only works if you can still make good decisions consistently.
I used to do that a lot, playing at multiple tables simultaneously. I don't do it anymore, though, because I prefer to focus on a single table where I can better evaluate my opponents' actions.