All retirement plans work under the same basic premise. You can pay the IRS now and then avoid taxes on the distributions later. Or you can avoid taxes now, but pay them when you take the money out. One of the advantages of the regular IRA and 401k plan is that the contributions you make are deductible now. You'll pay taxes on the money when you withdraw it.
Many people expect to be in a lower bracket after they're retired.
So they'd pay less in taxes. The Roth IRA is different. You contribute money that's already been taxed. All three programs share one advantage. Earnings on the money invested in the plans are not subject to taxes each year.