Because it allows you to shield that money's growth from income (e.g. most bonds, most reits) and capital gains tax (stocks, qualified dividends), something the government wants to limit for people in the upper brackets during accrual or retirement.I don't understand why IRA contribution limits apply to Roth IRAs. I mean, I know why TRADITIONAL IRAs have contribution limits - the money you put into a Traditional IRA is before-taxes, so the government has an interest in limiting how much money you can shelter from taxation. But Roth IRA contributions are AFTER taxes... so why bother with limiting contributions?
If they took away the ability to penalty-free withdraw your Roth's contributions, I could see them raising the contribution cap significantly.