This is a grinding stone...it has the bigger and the smaller one. The smaller one is use to grind on the bigger ones.
...it's important in the olden days till now...it helps and take us out of emergency, I doubt this recent generation can know his functions and what it's all about perhaps it's still present in some home most especially in the Africa home
However the function of the stone is to grind pepper, okra tomatoes, beans, potato e.t.c...it's mostly use if you're planing to grind anything in little quantities as it will prevent wastage