Mistakes and extraneous junk doesn't get erased, it just isn't referred to by any Region. As the SAW manual glibly points out, disk space is pretty cheap these days, and when you finish a project, you presumably back it up to a CD-ROM or some such, and clear it from your hard drive anyway. The obsessively neat, alas, may never quite be able to get used to non-destructive editing. This powerful editing environment takes some getting used to. In SAWPro, you have to learn to work with sound files, Regions, Multitrack Entries and Sequences.
Regions are the basic building blocks of a piece.
A Region is a user-defined section of a sound file. Once a Region is assigned to a track in the Multitrack View, it becomes a Multitrack Entry, of which any number can live on a single track. Several Regions one after the other on a single track form a Sequence. Read the online manual. Once you get it figured out, this arrangement offers incredible flexibility in putting together a piece.