401(k)s offer tax advantages, employer funded matching contributions, and convenient, automated deposits. Plus, the IRS discourages 401(k) withdrawals prior to retirement age -- a good thing for anyone who has trouble saving.

Still, there's a case to be made for investing outside your 401(k) once you've maxed out your company match. And that case is built on boosting your financial flexibility, both before and during your retirement years.

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1. To access better investment options

Investment options within your 401(k) are usually limited to 10 or 20 mutual funds, index funds, or target-date funds. Those funds may be solid, low-cost choices, or they may not be. If your 401(k) investment choices are sparse, you stand to benefit long-term by supplementing your 401(k) portfolio with other quality assets in another account.

That other account could be a Roth IRA, a traditional IRA, or a taxable brokerage account. All three accounts give you access to the full range of stocks, mutual funds, exchange-traded funds, and debt securities.

2. For tax diversification

Your 401(k) distributions in retirement are taxed as regular income. That's because you've been putting tax-free money in that account for years, and that cash has grown over time, without year-to-year tax implications. Once you retire, the IRS is ready to start collecting its share of your wealth.

From a numbers perspective, that tax deferral usually works in your favor, since you'll likely be in a lower tax bracket in retirement. But the numbers don't tell the whole story. Although you ultimately save by putting off your taxes until retirement, the bill comes due when you're living on a fixed income and, probably, worried about outliving your savings. For your retirement budget and your sanity, it's wise to keep your tax bill as low as possible once you've left the workforce.

You can do that by proactively developing tax-efficient sources of retirement income now. If you meet the income requirements, you can save and invest in a Roth IRA. Roth IRA distributions are tax-free as long as you've had the account for five years and you're at least 59 and a half years old.

3. To protect liquidity

In exchange for the tax perks associated with your 401(k), the IRS asks you not to pull your money out before you reach 59 and a half. To hold you to that deal, the IRS imposes a 10% penalty on early withdrawals.

If you are targeting an early retirement or otherwise prioritizing your liquidity before the age of 59 and a half, this trade-off isn't ideal. That's not to say you shouldn't save in your 401(k), but you don't have to hold all of your wealth in a restricted account if those restrictions don't align with your future plans. You can address your liquidity needs in your 40s and 50s with supplemental contributions to your taxable account, since there are no withdrawal restrictions there.

A Roth IRA is an option here, too. You can withdraw Roth IRA contributions, just not the earnings, at any time without penalties.

4. To ease restrictions, taxes on your heirs

Once you reach the age of 72, the IRS requires you to take taxable distributions from your 401(k) -- whether you need them or not. These are called RMDs, or required minimum distributions, and they're in place to keep you from deferring taxes on your retirement funds indefinitely. The amount you must withdraw each year is based on your life expectancy and the account balance at the end of the prior year.

When you pass away, any remaining 401(k) funds are bequeathed to your beneficiaries. Depending on when you pass away and the beneficiary's age, minimum distributions will likely be imposed on your beneficiary. Any withdrawals made by your beneficiaries, RMDs or otherwise, would be taxed as income.

Funds you save in a taxable account, however, aren't subject to RMDs. You can let those assets accumulate until you pass them on to your heirs -- who also won't have RMDs on those funds. That's an advantage over the 401(k) and even the Roth IRA. You don't have RMDs on your Roth during your lifetime, but your beneficiaries likely would after you're gone. Fortunately, those Roth IRA distributions are tax-free for your heirs.

In a taxable account, your beneficiaries don't pay taxes on the distributions themselves, but they would pay taxes on any realized gains incurred when they sell assets. Your heirs will get a break on the cost basis of those inherited positions, however. Your beneficiaries' cost is the value of the assets on the day you died -- rather than what you paid for them. For example, say you purchased 10 shares of Alphabet in August of 2015, for about $650 per share. And, hypothetically, let's assume Alphabet is trading around $1,700 when you pass away. Your heirs' cost basis on those shares is $1,700, not the $650.

Plan for any scenario

If you want to be ready for anything in your senior years, an additional investments outside of your 401(k) can help. A Roth IRA or a taxable brokerage account makes for nice, flexible companion to your 401(k) -- something you and your heirs can appreciate.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.