Yes, several of these CS2 case opening sites only accept skin withdrawals and deposits from your Steam account. You need a valid Steam account to use these sites, as all skins are funnelled through there.
Some of these sites are also connected to the Steam marketplace as that is where most skin buying and selling takes place. Websites purchase them through the Steam Marketplace to place them in their inventory, and users can then get these skins through cases, Upgrades, or more such games.