to be honest, the best answer to this question is "why wouldn't it?"
The earth was made from a spinning disk of dust, each of those dust grains would have had a random (within a certain distribution) rotation. The net rotation was unlikely to be 0.
Also, tidal acceleration from other celestial bodies makes the rotation speed unstable, so if it ever was 0 it was unlikely to stay that way. Though this is a small effect.
In my opinion, this is one of those issues that we really haven't answered yet. There are certainly a number of plausible answers ( that Google will find for you), but, they are all based on conjecture of one kind or another.
Most of those don't make a whole lot of sense to me -- stuff spins because it formed by spinning - yea, maybe. -- I just think we as humans are being a little bit arrogant to think we've figured out the formation of the universe and these related questions. We're just not there yet.
I think we'll have a better handle on this one in about 500 years. In the meantime, Capuchin's answer will probably be the best we can get.
Zazonker - all objects in space have an intrinsic value of angular momentum. It would be unreasonable to expect that any natural object such as the earth, moon, sun etc. would have an angular momentum of precisely zero. Hence, all natural objects in space rotate - some faster than others, and your point that we have to resort to conjecture as to why the earth spins at the rate it does is fair. Nevertheless, Capuchin was correct when he said that the best answer the OP's question is: "why wouldn't it?"
Well, perhaps. I don't want to belabor the point, I just don't feel that, "Why wouldn't it? is a good answer to a question. You could, for example, give the same answer, or similar words matched grammatically to the question, to every question. But, that merely throws the question back to the asker.
And, Capuchin DIDN'T just leave it with that non-answer. He followed it with a couple of sentences tieing it to one of the theories on how the earth was formed. You also provide complementary information to deal with the question. All fair comments and a decent discussion.
It is however, an area in which accepted theory has changed greatly over the past 100 years. I was merely trying to indicate (and perhaps, I didn't do it well) that I believe accepted theory will continue to change for some time. But, if the question relates to OPs homework, it is best that he answer according to currently accepted theory. It that sense, both of you have probably done him a better service than I.
So, actually, the only thing that you say that I disagree with is that the best answer to the question is, "Why wouldn't it?" -- simply because I don't consider that to be an answer to any question.
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