While it might be easiest to leave the account where it is, it’s possible that you are sacrificing flexibility and/or paying higher fees in exchange for the easier path.
Quite often, 401(k) plans (and other qualified retirement plans, QRPs) are restricted to managed mutual fund investment options. Managed funds often carry high expense ratios, often greater than 1%. As you know, if you’ve read much about index funds, it is possible to reduce most of your investing expense ratios to far below .5%, in some cases as low as .1%. Over the course of many years, reducing these expenses can have a profound impact on your investment returns.
For example, if you were to save even 1/2 of a percent in expenses, over 20 years this could compound to a 11.05% improvement in your overall investment returns. This also assumes that the new funds you’ve chosen will perform at the same rate that the funds you’re leaving behind would have.
It’s not a pure “no brainer” to decide to do a rollover. There could be compelling reasons to leave the money where it sits, such as if you believe the funds in your plan are superior to options that you could choose outside the plan (such as restricted-access or closed funds), or maybe you have access to investment advice from the custodian at no additional cost. In addition, if you left the employer during or after the year when you reached age 55, you might want to leave the money where it is until you’re at least age 59½ for flexibility in withdrawals without penalty – see this article on post-55 withdrawals for more information. There is also the chance that you could benefit by leaving the funds in the former plan if there is Net Unrealized Appreciation of your former employer’s stock that you intend to have treated by the NUA rules.
In general though, the flexibility to reduce your expenses by choosing any investment available is a pretty compelling argument in favor of the rollover.