One of the biggest benefits of participating in a 401(k) or other defined-contribution plan is that contributions can be deducted from your paycheck on a pretax basis, which can lower your taxable income for the year.
If you've been able to make significant pretax contributions over many years, and you've received generous matching and/or profit-sharing contributions (in addition to the benefits of compounding growth), chances are you've built a sizable nest egg.
