Assuming the plan accepts rollovers, and ignoring any pre-tax vs after-tax vs Roth issues:
A 401(k) plan (or 403(b) or other similar plans) may provide access to special share classes of certain investment options (e.g. mutual funds) that have lower fees than what is available to a standard retail customer (e.g. via an IRA or regular brokerage account). This is not always the case, though, and you need to evaluate the specific investment options available in your plan to see if this applies. If you are invested in a fund in your IRA that has a cheaper class available in your 401(k), you could save some money on fees.
The plan may also offer a loan option, with certain limits such as only allowing you to borrow X% of your balance. Ignoring the question of whether such a loan is a good idea, or better/worse than just withdrawing (potentially with an early withdrawal penalty) from your IRA, rolling an IRA into your 401(k) could increase the amount you are able to borrow.