Daily Free Spins have some of the highest bonus values you can find. This is due to the fact that you have to come back for them every day. A daily customer has high value for a casino and that's why they are willing to give you a big bonus for it.
How much the bonus is wordt depends on the slot you play, the value per spin, and how lucky you are in the end.
With a deposit of €10 you receive 10 x 20 (200) daily free spins on Book of Dead. The initial value of this bonus is 200x 0,10 = €20. This is already a really generous 200% bonus .
However while playing one of the days maybe you get really lucky! Say you win €300. These winnings are bonus money which needs to be wagered. After wagering you have €100 left. This is now your money, which you can withdraw if you would like. So instead of the initial value of €20 your spins might me worth much more. Please always read the t&c at the casino you are playing.