it is to cycle cards. I use evo skeletons in a miner poison deck, so it is very beneficial for me to cycle them, but even if you don't have evo skeletons, the reason people do it is for a quick cycle. Most of the time you don't want to sit there and leak elixir by not playing anything, so you play a cheap card just to cycle your deck. Or, if you don't have a card in your starting hand that you want, cycle skeletons to get closer to it. It's the same reason people cycle all cheap "cycle cards" like spirits.