The free spins won't allow you to win anything. It is just a promotional tool to draw you in so you play more. Its just like how casinos will givep layers free rooms and free food so they stay longer and play. For whales they give them the most ridiculous perks and even let them fly the casino's private jet just so they bet millions.
So these are just a strategy to make more player for them? We are never going to win those free spins 
You are correct winning on free spin is very rare. Plus you still have to do roll overs before you can withdraw if ever you win from free spin. Its just a marketing strategy to get new users.
Sometimes, a gambling website gives free spins to their members for reward programs or they are making an event with many prizes. That will attract many players to comes to the site because the gambling websites are making promotions in many ways, including in social media. I got to use the free spins in the past, but I never got win, but I never think seriously with free spins because I know that will be too difficult to win the money or the other prizes. I prefer to get a faucet than free spins because, with the faucet, we can get free coins every day if we don't have a balance to play.
Yeah, i have seen few gambling sites are giving us free spins and free satoshi. No doubt it is attractive way, but i think it is useless, if we are a player then we play gambling like a players, with free spins we can't win. It is seem like we play game, not gambling.