Online casinos hand out spins on many different occasions. You can get free spins:
when you sign-up
on top of your 1st deposit match bonus
as the only 1st deposit bonus (usually with a bigger bet size per spin)
from other casino promos
when a new game is released
from the loyalty program
Even when the free spins come with a deposit bonus, you can usually get all the spins with just the minimum deposit. Sometimes without risking any of your own money entirely. However, you still have the chance of winning real money.