There are multiple types of snapshot (e.g., snapshot replication, database snapshot, snapshot isolation level). I assume you mean database snapshots, which are designed to present a point-in-time, read-only copy of a database. Snapshots are especially-beneficial when you're using report suites that need a stable view of the data. As data changes, the original pages are added to the snapshot. This process, also known as "copy on right," can cause the performance of a database snapshot to decline slightly over time.
Platform Program Manager
Microsoft Project Team