Projection models are "vanilla PHP objects". Its purpose is to represent a stored document.
The projection is usually a transformation from a domain object (e.g. an entity) and therefor projections should be considered read-only and disposable, given they can be re-created / synchronized at any point in time from a source of truth (the repository).
A practical use case for projections are APIs, where each API resource is a so called projection, corresponding to its domain entity. It enables decoupling and thus optimized API responses.