Free spin indeed doesnt guaranteed a reward since you can literally earn zero despite paying big amount in worst luck.
Its high risk high reward since you are paying in lump sum in equivalent to 100 spin for a chance to enter a bonus round and win big.
If you play manual 100 spin theres no guarantee you will hit a free spin but you will recover back some of your losses from normal spins.
If we get free spins through manual spins, there usually aren't any big multipliers, like I experienced in the Olympus slot. However, there could be a big multiplier, or we might just win a few dollars. However, it's quite unusual for a free spin to end with a zero payout at the end of the spin, which is certainly disappointing. Depending on the game type, sometimes there's no buy free spin button, so every free spin we get could result in a large multiplier.
I agree that playing 100 manual spins usually results in a free bonus or not, but 100 manual spins usually result in a big win or mega win, so the money we spend on normal spins usually increases or doesn't incur a significant loss.
Slot games have a high profit margin. If we get a free spin bonus from this alone, we're saving money, as if we were betting 0.2, we would have paid $20 to buy free spins.