Why are there three branches of government in the US?

Answer 1

It is an easy way to control the government because each branch knows what they are supposed to do and the burden of responsibilities is not on just one group. Along the same lines, the three branches help to insure that no branch, and no individual person, can get too powerful. For example no president, however powerful or brilliant, could take powers that he/she is not given in the Constitution. The framers were keenly aware of the kind of government they did not want-- a monarchy. Yes, there are problems with the structure they devised for us, but all things considered it has served us very well indeed.

You should never offer this to your teachers as a formal response to governmental questions, but in a very real sense our government has four branches and not just three. The People, as embodied by the sovereignty of each of the states, really are the ultimate civil authority in the US. The People give powers to the government, and they reserve to themselves any powers not specifically given. It is NOT the government that 'gives' certain rights or powers to the states. Also, it is The People and The People alone, without permission of or required collaboration with any government, that have the authority to amend the Constitution. And no individual or branch of government can veto any change made to the Constitution by The People.
The three branches of the U.S. Government (Judicial, Legislative, and Executive) are needed in order to keep balance of power using the system of checks and balances. One can't do anything without the approval of the other branches. This is why the President can't make laws just to please himself.

Answer 2

to have a balance of power