Summers and weekends give children time to be with their parents and do those necessary growing activities. Sometimes I wonder if the public schools are nothing but inexpensive babysitters for some parents. Christmas break and summer give kids time to just be kids. It gives them time to use their active imaginations how they like, without the confines of assignments. It gives them time to visit grandpa and grandma, who are almost always an excellent source for learning.
Summers give high schoolers the chance to work a job and budget money.
So it might be a mundane job, but it teaches work ethics and interaction skills not taught in formal education. American schools are in danger of teaching too many things too soon. Many foreign countries try to track kids onto programs for a field, but American schools try to teach a broad education and let the child find where his or her skills are strongest. We shouldn't be teaching keyboarding to kids in kindergarten. Being a well-rounded person distinguishes the typical student of American schools from those in many other countries.