Calcium carbonate has traditionally been a source of CaO in raw glazes. CaO is not an active melter below about cone 8, so particle size can make a big difference in its willingness to enter the glaze melt. A 325 mesh material could create a glossy glaze whereas a 200 mesh could create a silky matte, exclusively because of the difference in particle size.
In low-fire bodies, calcium carbonate is sometimes added in small amounts as a filler to reduce fired shrinkage and act as a whitener. It is also common to see 5% whiting included in porous earthenware body recipes to prevent moisture expansion.