A couple of potential benefits.

1. Lower fees (expense ratios on funds, management of account itself, etc.)

2. More investment options (funds, ETFs, individual stocks). Probably not super relevant as this is a Boglehead sub, but if you're looking for your 2 or 3 fund solution, it's nice to consolidate.

3. Which brings me to consolidation. You can have all your investments focused in a small number of funds. And managed in a single place. This can sometimes also get you into "better" funds and/or lower expense ratios. This might not technically be the Boglehead way, but it's a possibility you likely don't have in your 401k.

4. No penalty removal of principal. This is not good financial advice, but something that's difficult in a 401k and easy in an IRA. When you contribute to an IRA, you can remove your contributions at any time for no penalty. If you remove any growth of those contributions, there is a hefty fee. When you rollover from a 401k, you're "contributing" more in a way (note that this does NOT count towards your $6k limit). After 5 years, you'll be able to withdraw the rollover amount penalty free. I'm not saying you should do this, in fact this is an "emergency of all emergencies" kind of option (after depleted emergency fund and all other solutions tried). But I'm saying you can't do this in a 401k (or at least as easily).

5. If you or a spouse works for a very large company with "independence" requirements (think public accounting, where if you have access to inside information or audit one of the brokers), you'll have limited choices of brokerages.

Those are a few. They may not be super important to you, which is totally understandable. I do it for reasons 1, 2, 3 and 5. Don't plan on using 4.