Suribachi and surikogi are Japanese version of mortar and pestle with ridges. They are one of the fundamental gadgets in Japanese kitchens.
The Japanese often use it to grind sesame seeds or make tororo smoother after grating. But they are also perfect to mash potatoes or other root vegetables.
Other than use as a kitchen gadget, a suribachi can be used as a nice presentation bowl. It is convenient that you can bring the suribachi to the table, after making the side dish in it.