Builds discipline and creates the habit of doing homework.
If you travel to lets say, SE Asia, 6 year olds are already beginning to do algebra where the 6 year olds here in America are coloring inside of coloring books.
Homework for 6 year olds? What does it accomplish?
In my opinion I believe it helps narrow down what a child's strengths and weaknesses will be academically going into the future which in turn will help with the child's development in finding a suitable study for later in life.
Giving homework to young children helps them exercise their minds so they can grow faster and stronger. It also helps prepare them by developing the self-discipline they will need in high school and later on in life.
At that age the homework shouldn't take them more the 15 minutes to do. It is a way for parents to know what their kids are doing, as well as developing the habit of doing homework.
Probably nothing at this age other than getting them used to doing homework in the grades to come, and may be more of a marker to see what these kids are retaining nowadays