After you turn 70 ½, you must make required minimum withdrawals from a 401(k). That means you can't leave the money in there, growing and growing, as you might like to do if, for example, you think you’ll live a really long time or plan to leave some of it to your heirs. If that's your situation, you're probably better off with a Roth 401(k).
When you eventually make withdrawals from a traditional defined contribution plan, you'll have to pay regular income taxes on the money you withdraw - whether the money came from your contributions, dividends or capital gains. It will be taxed at your income tax rate at the time you withdraw it.