I didn't know what it was for when I first started, so I had a few stacks of bones in my chests. Luckily, I am a packrat.
I wish the trees dropped more sapplings. Sometimes I get 1 right when the tree is first grown, but I might only get 1-2 when it is cut down. Growing a jungle tree from just one sappling sometimes gives nothing as far as saplings. I do like jungle trees best, though.