Why would someone lock up money by rolling it into a 401(k)?

TL;DR

Why would someone lock up money by rolling it into a 401(k). Dear Dan, I understand why someone might roll funds out of a 401(k) plan, but why would anyone under 59½ ever roll money into a 401(k) and lock it up. —Wondering About Withdrawals Most Read from MarketWatch - My mother, 91, has dementia.