Buying bonuses sometimes forces yourself to spend faster than the capital we have, do you think like that, it could be that 4 purchases have no profitable return.
But, it's save you more.......
If the section you are playing are on the cold section. Let's say more than 500 spin without bonus, and you spend 0.2$/spin we can guess around 20-30% your money back from the normal spin hits. Meaning you spend 70-80$ for the normal spin without any bonus at all, by bonus section you save your self with the bonus 2-3x times buys.
It's not about the profitable return, but saving cost spending for the spin.