I put these on my son when the days are just not warm enough for just his onesie, but not cold enough for pants.  They are a little long right now, but I kind of like it that way so I can cover his heel. (he is 5 months old).  I love these because sometimes while learning to scoot and crawl he will scratch his legs with his toenails.  When I use the leggings he can't do that, but still had his feet available for gripping.