Though Social Security helps millions of seniors stay afloat financially, living on those benefits alone could mean winding up cash-strapped in retirement. With an average monthly benefit of just over $1,500, Social Security was actually never designed to sustain seniors in the absence of other income. As such, it's important that you line up some additional income streams outside of those benefits, and a good one to consider is a Roth IRA. Here's why.

1. You can fund a Roth IRA at any age

Many seniors opt to work during retirement, whether to generate income or to give themselves something to do with their free time. If you choose to go this route, you should know that you'll have the option to put those earnings into a Roth IRA. The great thing about these accounts is that you can contribute at any age, so if you decide to work more so for the purpose of alleviating boredom and don't need your full paychecks to live on, you can stash that money away and let it enjoy the same tax-free growth your savings are privy to.

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2. Your withdrawals won't cause you to pay taxes on your Social Security benefits

If Social Security is your sole source of retirement income, you'll probably collect your benefits in full without being subject to federal taxes. But once your earnings exceed a certain threshold, taxes on Social Security income begin to apply.

To see if you'll face taxes on your Social Security benefits, you'll need to arrive at your provisional income, which is your non-Social Security income plus half of your yearly benefit. You could be taxed on up to 50% of your benefits if you earn between $25,000 and $34,000 as a single tax filer, or between $32,000 and $44,000 as a married couple filing a joint return. And if your provisional income goes beyond $34,000 as a single tax filer or $44,000 as a joint filer, you could be taxed on up to 85% of your benefits.

The good thing about putting money into a Roth IRA is that any withdrawals you take from that account won't count toward your provisional income. And that might leave you with more money from Social Security.

3. You get plenty of flexibility

The Roth IRA is the only tax-advantaged retirement savings plan that doesn't impose required minimum distributions, or RMDs. That gives you a lot of flexibility with your money. You can leave your account alone for as long as you'd like so your savings can continue enjoying tax-free growth, and you can leave some money to your heirs if that's something you can afford to do.

You should definitely plan on having access to some retirement income outside of Social Security, and while that income doesn't have to come from a Roth IRA, it pays to open one and contribute steadily during your career. While it's true that a Roth IRA won't give you an immediate tax break -- your contributions will be made with after-tax dollars -- the benefits you stand to reap in retirement will more than make up for that.