Generally, you can only withdraw money from a 401(k) plan upon attainment of age 59½, termination of employment, financial hardship, or termination of the plan (there are a few other special distributable events like QBADs and QDDs as well).
IRC 72(t) imposes a 10% penalty on any distribution from a plan, with exceptions for distributions made after age 59½, or after the death or disability of the employee, or if termination of employment occurred after age 55, or various other reasons.
So it's not that the plan allows a penalty-free distribution at age 55 - it's that the plan would generally allow distributions after termination of employment (at any age), but if termination of employment occurred after age 55, then the 72(t) excise tax does not apply. The plan doesn't need to specifically allow this or really address it in any way; it's simply a consequence of the way the tax code is set up. However, a 401(k) plan may not permit distributions at age 55 in the absence of another distributable event.
To get back to the original question, I personally see little to no point to defining an early retirement age in a typical 401(k) plan.