Forgot to say, big difference is in a drawgrid you can draw symbols, which you can't in a plain grid.
A normal grid is most useful when doing something like Csv, where you don't need symbols as you want data to be readable / usable everywhere and do copy / paste etc.
A drawgrid also somehow feels a bit clumsy to me, something to do with scrolling.
Edit: it's because you have to drag the slider quite a bit to have effect, i googled it a bit, it seems this is a bit of an oversight in that control. I looked at hacking into it but it's rather complicated. The filemanager in pd is also a drawgrid btw.