Comment by saltminer

Roaming. Although a desktop might not usually be moving while it's on, a laptop user might move around the house, conference room, or lecture hall, and switching APs could result in a stronger signal. It's better to switch before your signal gets weak/drops to minimize disruptions during BSS transitions.

On an unrelated note, in my experience Apple devices are consistently the most frustrating devices when it comes to wifi. They are insanely picky about what networks they will connect to, and they are never consistent.