It is a good practice to regularly track the amount of storage space that the site collections consume and the pattern of usage. This assists in capacity planning and at appropriate intervals, helps you allocate or reclaim storage space.

The  provides information such as the amount of disk space that all the document libraries, individual documents, or lists in a site collection consume. The Storage Space Allocation report is available at the top-level Site Administration page under the Site Collection Administration section.

You must define storage quota for the site collection in order to be able to view Storage Space Allocation. If you didn't define a storage quota, whenever you tried to view Storage Space Allocation report you'll get this error:
"The storage space allocation page cannot be used for sites that do not have a storage quota defined".