Why is the base necessary when building a patio out of irregular natural stone?

I intend to build a stone patio approx. 20' X 20' in a frost region. I've researched the varieties--pavers, patterned natural stone, etc. All types require first laying a base-- from a couple inches of sand in some cases, to 3-4" of an aggregate base, then topping with 2" of sand/screening material in other cases. I, however, am envisioning placing irregular natural stones (sandstone or bluestone) with spacing of approx 1-2" with moss or other small ground cover plants in between the stones. Why is a base necessary? If my ground is already level, why can't I just set the stones down or dig out the depth of the stones and set them level with the ground? Is is a drainage problem? I'm very curious. Thanks in advance.