I have always thought it was to consolidate what has been learnt in class and perhaps stretch them slightly to stimulate them to learn more. However dd's recent homework has made me sincerely doubt this. It seems to be to test the skills, commitment and financial capabilities of the parent, be so far above their current ability level it destroys their confidence and leads to hours of tears.
For example dd had to do a project on an Olympic athlete. They wanted a full essay and diagrams showing their career and training so far. I got dd to had write a short summary based on what we had sat and read together. She was marked down because it was compared to work that adults had spent hours on with no child input. What does the child gain from that.
I am perfectly capable of doing that but why? It's not my homework. Also if homework is hrs a night at 6 what is going to be expected at 16?