I think that's why it is called "free" spins, because you will nothing really much from it. The purpose of that bonuses is to encourage players to play more. Though, I knew some people who won using those free rolls, ranging from $20-$40, but then again, it's not that much really because eventually, that winning amount will get drained.
I'm not really a huge fan of slot games, because your luck depends on how many players have been playing at the same time lol. If a lot of people are winning during the time you're gambling, then your chances of lossing is greater. That's just my theory.
I've won using free spins before but I still won't encourage anyone to depend on bonuses from casinos because most of the time the opposite happens, it is free bonuses, something you don't pay for, lower your expectations.
Free bonuses are a good way to bring in new people, to atleast have some experience about how the games feel like, they are not giving up free bonuses to make people win free money, this is similar to using demo account on a trading platform, it is all about the feel and the experience.
The less expectations from bonuses the better, the last time I won using bonus spins from slots was not expected, it just happened, although I have to meet up with the casino criteria to withdraw but that's it.