I've mentioned this before across HN, but I'll mention it again - human-to-human interaction is critically important for young children.
You basically have to teach young kids how to act like a human being. Kids will get this teaching in daycare and through their parents, but that alone isn't enough and it is important to reinforce this through schools.
People often forget this because you don't really remember it and then you end up making assumptions on how you act now - not realizing that a teacher may have taught you when you were younger.
I think the best example of this is handing out worksheets to kindergartners. An average person may think that you just give them pencils and paper. An experienced teacher knows that they have to teach the kids where to write their name, how to follow along the line, to read instructions, flip the page over to check for more, etc.
This is practically true for all children. Again, many may argue that you can receive this teaching through your parents and day-care, but once a child gets old enough they are spending most of their time in school. Think of it like language learning. You'll learn more of the language if you are being taught it all day, rather than only at home with your parents.
You also experience more interaction with kids your age at school. There are many situations that come up where a teacher guiding you is important, and these same situations may not come up at home (especially if you are a single child).
I do think after a certain point you can rely on technology more for learning. I just want to emphasize that having a human teacher trained for working with children is critically important for the growth of a child.