A snapshot creates a copy of the current project. The snapshot is frozen, it cannot be edited. But you can reset the project to the exact state of any existing snapshot. This is useful if you want to preserve the current state, for example because you want to document how the planning and state of the data management evolves with time. Or you want to make a substantial change in the DMP and want a mini backup.
Some funders like the ERC (H2020) expect to be presented with a new DMP after each project stage. Snapshots allow preserving the state of the planning while keeping the DMP a “living and changing” object.