Snapshots is a great feature within Microsoft SQL Server Reporting services, to see any of your reports at that point in time. The same report cosmetics might have changed recently as part of the change request and the data will be changing everyday. How did the report looked in terms of look and feel and the data that was latest same time last year? For this question a simple answer is the snapshots. Another simple example is your photo that was taken today and comparing the same with your own photo that was taken 5 years ago. Now do you see the difference? Yes, this is an amazing option we have within Microsoft SQL Server Reporting services for your reports.
By running these snapshots of your reports, it reduces the load on the databases directly, but any amazing feature may have its own negative side as well. It is always recommended to keep less than 8 to 10 snapshots for a report in the SSRS and download the rest as a PDF and delete the previous snapshots as the data and the layout are storing in the Report Server DB and this will unnecessarily bloat your SSRS system databases. Below is also a screenshot that explains more on Snapshots. You can also view a quick demo on snapshots in this link.