They come from nearby deaths

The reaction trigger matters. If your reaction has already been spent, you may lose the opportunity to capture that departing soul.

There is a carrying limit

The Phantom cannot accumulate unlimited trinkets. The maximum number is tied to proficiency bonus, so deciding when to spend one matters.

Passive defensive value

Possessing at least one trinket provides the defensive benefits specified by the feature, including improved resilience on death-related and Constitution saves.

Information utility

Destroying a trinket can let the Phantom ask its associated spirit a question. The spirit knows only what it knew in life, so this is information gathering rather than omniscience.

Fuel for Wails

A trinket can also be destroyed to use Wails from the Grave without consuming one of the normal proficiency-bonus-based uses. This converts exploration and encounter history into later combat resources.