One good reason is so you can have more options on what to invest in since 401K plans are usually limited.
When you leave a job you have 3 options :
1) leave the 401K as is IF the plan allows you to stay ... most do not.
2) take the distribution and pay the taxes + penalty on the withdrawal.
3) roll it to an IRA ... make sure to do a trustee to trustee rollover ... if you touch the money you only have 60 days to make the roll and the employer will withhold at least 20% for federal taxes which will not be able to be rolled thus it is taxable.
