~ Don't expect free spins will give you a good reward since that was free after all.
But if you are playing slots with your own money and trigger the free spin while you are playing, expect good rewards from it and even big multipliers and bonuses.
I'm not aware that there are sites that give free spins, I mean literally free? Am I right about what I understood in your post?
Actually, there are many gambling sites that are giving away free sips, and those spins are "literally free": whatever you win from them is yours with no additional wagering requirements. Check out twitter accounts of the established gambling platforms, subscribe to their telegram channels, Reddit and stuff. As I said above, you won't make a living from that, but it's a nice bonus.

Is it possible that the free spins is made available to only those who have spent some of their cash on the platform?
With a free spin or a wild one time try, people have made fortune. So, it is very possible to win with a free spin. My concern however is that most of the free spins may be too small compared to an actual win with real stakes on ground.
Making a free spin nowadays has plenty of rule and terms and conditions that is backing it and we won't be able to make a big winning from a free spin since different casinos do set a limit. I don't think we could make a lot of money from a free spin. Since the tittle has really explain what it means and shows that we it could give us some few dollars but can't make us rich.
There are some casinos that likes giving free spin to new users especially after there first deposit which look like a way to make there new users to become attracted to the casino mostly when they win some dollars with the spin.