Most people that work at a job that offers a 401k will invest at least some of their money. This is good because as soon as they retire they might find that things like social security and other forms of income do not provide quite enough to meet their standard of living. They have to supplement it with the savings they have accumulated during their working years. While having a 401k while working is a good thing, as soon as a person retires or switches jobs, they should look into completing a 401k rollover to an IRA in order to keep their money as close to them as possible, and have access to it whenever they want.
When money is left in a 401k at an old company it will remain under the fiduciary responsibility of the old employer. This means if the employer decides to go with a different manager, the money will move along with it. People generally want as big of a say in their money as possible, so by doing a 401k rollover to an IRA as soon as they leave, they are keeping the money where they want it, not where their old boss wants it.
As soon as the money is out of the 401k a person has several options. They can leave it invested as it is, switch it to new investments that they think will do better, withdraw it, or convert the traditional IRA to a Roth IRA, pay the taxes due, and not have to worry about the required minimum distributions that will come due as soon as they hit 70 years old. There is no reason to let money just sit at an old job when it can be transferred easily, and many companies allow old 401k money to be rolled into a new 401k. With so many options, why would anyone not do a rollover?