Oatmeal soap. What's the advantage? What other grains get put in soap and what do they do?

I’m staying at a hotel that supplies oatmeal soap in the bathroom. This is new to me – almost every other soap I have ever used has been, well, soap.

What’s the advantage to oatmeal soap? Does it help the skin? Where to the little pieces of oatmeal go? Are other grains used in the manufacture of soap? What do they do?

I guess what I am asking is – is this just a gimmick or is there some positive effect of oatmeal soap?