YES.. it is for 2 different things....
#1 grit stays in the crop/gizzard to help grind up food... depends on where you live.. i personally never have to offer grit.. i live inthe desert and my chicken freerange on sandy soil..... if your chickens are locked in a coop/run or you live someplace where there is not alot of sand/small rock, you will need to offer it to the chickens... just find or buy a bucket full of sandy/rocky soil and dump it n their coop/run/area
#2 oystershells is for Xtra calcium... i have allot of chickens.. i toss about 2-5 lbs of oyster shell into their area about once a month..
50 pounds of oystershells lasted me about 1 year