Elemental expeditions
Expeditions are pet work: you send a pet to an elemental region, it is gone for a while, and it comes back with materials. It opens at level 35 and is the point where the pet collection stops being a stat bonus and starts being a roster you assign.
How it works
Each region is elemental, and which pets are eligible for which route depends on element and development, so a broad collection opens more routes than a single strong pet. Attempts per day are limited and a pet on an expedition is unavailable until it returns. The timer can be skipped, which is the only reason this system ever competes with your combat plans for the same pet.
Best practice
Decide who your combat pet is before you send anybody anywhere. Expeditions reward breadth, and the second-best pet of four elements is a better expedition roster than one maxed legendary, so build the collection sideways for this system and vertically for the pet you actually fight with.
The common mistake
Sending your equipped pet because it is the strongest. The reward is materials; the cost is going into your next dungeon attempt without the pet the build was gearing around. That trade is almost never worth it, and it is the one mistake this system makes easy.