SQL Server: why to use a backup device?
I prefer one file per backup


DB_FULL_yyyymmddhhnn.bak
DB_DIFF_yyyymmddhhnn.bak
DB_LOG_yyyymmddhhnn.bak


I don't want to:


query a backup device to see what is stored there
copy an entire backup device to get one database
FTP or otherwise shift more I need around the place


If you have TB size database, partitions, filegroups and want to do partial backups/restores then these problems are multiplied. In a DR situation, I want everything simple and clear.

Most (large and small) shops I've been in do this: it is simpler to deal with self describing files.

The "device" concept goes back to Sybase, SQL Server 6.5 and earlier. Here you had disk devices too Separate MDFs and LDFs were introduce with SQL Server 7