It's kind like the president in the US. He can make come up with a plan but the three Branches of government have to okay it. The reason for Parliament having the real power is so that one person doesn't control everything. That makes the king and queen the face of all of the government choices but they are not the one who picked them. Parliament is made up of a lot of different people and that helps when it come to making choices because everyone has there own opinion.
I hope this helped
