From the MSDN an article on External Storing of BLOBs in WSS.
“Typically, as much as 80 percent of data for an enterprise-scale deployment of Windows SharePoint Server consists of file-based data streams that are stored as BLOB data. These BLOB objects comprise data associated with SharePoint files. However, maintaining large quantities of BLOB data in a Microsoft SQL Server database is a suboptimal use of SQL Server resources. You can achieve equal benefit at lower cost with equivalent efficiency by using an external data store to contain BLOB data.”
You can find this article here.