This plan did a number of things for them:
1. A salesperson could relax and know that the base salary is safe even if it's a poor month.
2. It incentivizes the sales team to sell more. The more they sold the more they can make via an exponential factor.
3. Corporate finance realizes that if they were just hitting their break-even number they didn't have to pay bonuses and we would easily cover overhead and hit all margin targets.
4. Everything after the 120% was effectively gravy, so splitting that up only increases their bottom line at a slower rate and made their sales team that much more engaged.