Why a tiller? . . . I suppose the most correct answer is "because we are guys and it is a power tool."
A more correct answer might be that even with my rock hard clay soil, if the moisture content is right, I can turn it into something that resembles plantable soil in just one pass. Or that I can incorporate manure, compost, sand, dirt, moss, etc into my clay to improve it to turn it into something that is not only plantable but reasonably healthy soil. Or that I can break up sod and turn it into garden or planting beds for the lovely Mrs_Bob in one easy step. Under SOME conditions you have to go very very slow. Under SOME conditions you can move along at a reasonable speed. It really depends. But if you consider a plow, you can go faster, but a plow typically breaks the soil into fairly large clumps and clods, so you then have to go back over it with a disk or drag or both and you end up making several passes and your total time might end up being about the same as if you simply used a tiller. And a plow and a disk still might not give you the fluffy soil that a tiller is capable of producing. I think that a tiller is useful for smaller patches, I'm not sure I'd want to do several acres with a tiller, at least not without a large commercial uint. For those of us with small tractors and small plots of land, who pretend to be weekend farmers, a tiller is simply and easier tool to use to achieve the results we need to achieve.