[D-fendr]

Don't have time before bed to give this post it's deserved attention, but I'll give the quick answer to the first question:

Why does it require it? (first cause, outside..)

To avoid the problem of infinite regress.

[kosta50]

To avoid the problem of infinite regress

That was, I believe, the argument of St. Thomas Aquinas, which is not without a flaw because it relies on unwarranted assumpton that deity is not subject to regression.