This metric is useful if you want to know exactly how much physical space a particular table is taking up, including the size of its indexes. If a database is growing quickly within a short time and you suspect a certain table is responsible, you can monitor its actual size, or the rate at which it is increasing.
If you have a particularly large table affecting performance, you may consider:
- Carrying out index maintenance – see http://www.mssqltips.com/sqlservertip/1339/sql-server-index-maintenance-checklist/
- Data partitioning – see http://www.mssqltips.com/sqlservertip/1200/handling-large-sql-server-tables-with-data-partitioning/