Seems like the more notice the departing employee gives, the longer the employer will delay the search for a replacement, probably for economic reasons -- especially if it is a relatively small employer. Why should they have two people on the payroll for one position?
Saw an employee give a year's notice once, and the employer waited until 8+ months to start looking for a replacement. They had to hire and fire one, then hire another (well these folks are not very good at picking people).
While I believe the departing employee intends to ease the employer's task of finding a replacement, the employer sees it as extra time to convince the employee to delay their departure, deferring the expense of conducting a search for a replacement.
If the employee got hit by a beer truck and never went back to work (morbid, but it does happen) the employer would figure out what to do.