James Anderson got most of it. I just want to add one more.
Purging is a huge reason we are working on Partitioning.
If you indexes are aligned, then you can do partition switching. For us that means that we can take older partitions and just switch them out to different table and then drop that table.
The actual switch allows me to remove over a million rows in less than a millisecond.
There are trade offs (some significant ones), but deleting data for large transaction systems is sometimes a very hard story.
My company has a SAN and the System Admins will not even hear of allowing storage anywhere else.
Despite the fact that my partitions will be on the same disk, I still am currently working in implementing it just for the data purging story alone.