They're superfluous, educationally speaking.
Yes, I just said that.
Every Single Thing your child learns in elementary school they will learn again in High School, when they have the ability to abstract, so they can get the big picture much more easily, and then again in College. That includes all of the social stuff, all the academic stuff, all the cultural stuff. Everything. They will make new friends both of those places, too, on the whole (very few people stay really close to their elementary school buddies for their whole lives. It happens, but not often).
The only thing they won't get again unless they want it is arts and crafts.
Even if they miss reading and math facts, the two really important things they learn in elementary school, provided they don't have a learning disability, they can catch up on really fast when they're older (although they might curse you for the joys they missed because they couldn't read well when they were young--being able to read and do basic math is a blessing to children that gives them a great deal of confidence, independence, and a means to satisfy their curiosity and feed their brains, and, when paired with writing, gives them a way to express themselves).
And here's the other secret nobody's telling you: Their grades don't count for ANYTHING until high school, and then they only count toward college acceptance. And even then, if they do a year or two at a community college and get good grades, the high school grades are forgivable. And those standardized tests don't count for anything, either, even in high school. The only test that matters is the SAT/ACT, and even those are becoming less weighted in the overall admissions scoring.
The most important thing kids need to learn is how to learn, followed closely by creativity in solving problems, how to express themselves effectively, and how to interact with society in a responsible, meaningful way. They also need to discover and develop their own talents and gain some confidence. And here's the really big secret: all of these are best developed OUT of the classroom! In fact, they can all be developed very effectively by sending your kids outside for some unstructured play time, and research is starting to emphasize this.
So I'm very open about not being an unschooler, but I do think homeschooling moms (and all moms) should stop worrying so much about those "educational gaps" the schools are telling us we are condemning our children to, and whether or not their 5 year old is learning everything he needs.
At least, until the kid hits high school. And then you have to decide what you care about.