Sometimes it may be down to personal preference, people may prefer natural fingernails and thumbnails, but like to have their toenails painted. It can also be an easy way to accessorize the feet, and also painless since having tattoos on the foot can hurt a lot, since it's a bony area.
Also, there's the issue of nail varnish chipping. It tends to last on toenails much longer since people obviously use their hands more than their feet. That plus in some jobs, it may be for hygiene purposes, like if someone works as a chef so it won't chip off and get into the food, or if someone works with animals so the animals won't eat the chipped nail varnish, since that can possibly make them ill.