I felt a bit silly, because getting free spins is cool, it's usually exciting, and I've won some good big wins a few times.
Has anything similar happened to you?
I remember when I was a participant in one of the signature campaigns that gave free bets on the forum, I can't remember that particular campaign but I did win several times some cool cash from the free bet I got. In USD it mightn't have be big but in my local currency that was more than millions and this didn't happen just ones. I have used similar free bets especially spins to win some cool cash but if I'm being honest, I have lost more times than I won with this free bets although I didn't think about them that much since the bets were free and not me losing real money. I tend to take more risk with free bets than I'll do if I had used my money.
Judging from the narrative of OP, he is probably talking about the free spin from the bonus round that usually get if you hit certain number of scatter on a regular bet.
It usually have higher chance of returning big profit since the profit mechanism on this bonus round has special effect that boost winning percentage on each spin.
The free spin given by the casino which is just a regular spin is just a normal spin that is literally free. I do get a lot huge win too on that kind of free spin by hutting bonus round.