Bonus buys are nothing but another form of gamble. Based on the game volatility it could pay anywhere from 0x to 1millionx.
1. In most of the bonus buys you get some of the money back, without losing all of it. Games like Sweet Bonanza, Money Train pay you back like 40-50% back on consistent basis.
2. You don't have to hunt for long time to get bonus. For example it's very tough to get a bonus on MT2, instead you can directly hop into bonus buy with 98% RTP (higher than the regular game).
3. You could 2x 3x or even 10x your money faster instead of grinding for hours and losing it all eventually.
1. Even though most of the bonus buys pay back a majority of money back, in the long term it is bound to the game RTP and Volatility. Some games like Deadwood could give 0x or 1x several times in a row.
2. It has been proven time and again that base game is always better than bonus buys, but you have to choose slots that have good potential in base game as well like Gems bonanza, Gates of Olympus, Buffalo king, etc.