At that point there will only be three potential workers to provide Russell's Social Security payments. Russ would be foolish to solely depend on those three people to pay enough to fully support him on Social Security. Naturally, he'll want to accumulate money that can be used to pay living expenses when he's retired. Perhaps he'd like to reduce his taxes today. And he might want to have access to his money for extraordinary expenses before retirement.
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. 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.