To illustrate how it works, consider a dentist in her peak earnings years, with $500,000 of income. She's married and her children are out of the house. She is in the 35% federal tax bracket, biting deep into her income.
Because your contribution is defined — but not your retirement benefit — DC plans pose less financial risk to employers and much more popular with large companies. Guaranteeing a defined benefit on a retirement plan is riskier, so Uncle Sam imposes much higher contributions and more elaborate rules on a DB plan than on a DC plan.
Twenty percent of $315,000 works out to a deduction of $63,000, placing her taxable income at $252,000, firmly in the middle of the 24% bracket. The taxes are a lot lighter than if she hadn't taken steps to whittle down that half a million in income, and she has socked away a large defined benefit for retirement.