And without getting into the varieties of hiking/trail running/etc. You can easily encounter situations in that genre alone that necessitate 2 or more. Waterproof or not, trail running vs hiking vs thruhiking, the list goes on and on.

Editting to add: this isn't unique to hiking either, you'd find similar variations in runners too. Endurance, sprint, or agility (among others) running are all different enough you'd have genuine performance and health reasons to separate the shoes. Obviously hardly anyone will dedicate to that many unique activities, but some people really enjoy them so it's worth explaining.