They do the work, so you don't have to. Your plan may also include funds that aren't actively managed, such as index funds or funds of funds, or options other than mutual funds, such as a company stock fund or a commingled pool. Please see your plan materials for more information. But for those unexpected circumstances that can arise, many plans allow employees to dip into their account balances before retirement. In addition, as long as you repay your loan on time, you won't be subject to withholding taxes or penalties, as you would if you withdrew from your account before retirement.
Withdrawals are a different story.
Different plans may allow you to take withdrawals for different reasons. The most common withdrawal type for active participants is the hardship withdrawal. According to IRS regulations, to qualify for this type of withdrawal, your hardship must represent an immediate and heavy financial need and there must not be any other resources reasonably available to you to handle that financial need. Some plans also allow hardship withdrawals for other reasons. Check with your benefits department.