There are people we want our kids to read about from a young age. Great Christian saints, thinkers and leaders who demonstrated a commitment to excellence, explorers who explored, inventors who invented—any good role model for impressionable young people is who we want them reading about, and picture book biographies are a great way to introduce them.
Sure, high school students will get more out of a text-based treatment of the life of Abraham Lincoln, but 2nd graders won't. They want pictures to draw them in, to help them see the same world Abe saw, to give them a sense of splitting rails or debating Stephen Douglas right beside him. If it's Lewis and Clark they're learning about, if they can see the pine forests and the bears and the Indians, they're far less likely to forget the story of the Voyage of Discovery than if they're just reading it.
We aren't talking about books that take liberties with historical fact, or that emphasize pictures at the expense of reality. Plenty of kids' book authors seem to think that truth is a moot consideration when writing for the chronologically impaired, but we understand just the opposite to be the case: we need to be particularly careful about what our children read when they haven't even reached an age to discern between fact and conjecture.
Nor do we see pictures as somehow better than words. The Bible wasn't written in visible images, and when you're writing about real people who really lived and did real things, you can't rely solely on illustrations to communicate the facts of their existence and achievements. We're only saying that sometimes its better to show what you're tyring to convey, rather than relying wholly on language.
It's not essential that youngsters know all the ins and outs of history. At the same time, when they do need to study figures and events of the past, if they already have some idea of what was going on they'll have much less difficulty putting the pieces together. Introducing them to important and fascinating people through text and pictures is a much better way to get kids thinking about history, and one likely to help them retain the information rather than forgetting it as soon as they finish the last page.