Why does the United States have three branches of government?
It is not so much that the US has three branches, as that thinkers in the mid 18th century decided that any government naturally falls into three branches. The writers of the Constitution had read and contributed to these thoughts. So when they had to form their own government, they split it up in this form. All governments have these three functions. But in the ones that have grown up organically, like the British system, the functions are all muddled up, so that some people have influence in more than one branch. Without approving of all the details of the implementation, I think that the division into well separated branches is an excellent thing which has worked well for the US. I think that other countries (such as Britain) should look to moving their systems in that direction. Not necessarily in a big jump, but when changes are made, to ensure that they increase the separation of the branches.
