Almost every DWG contains unused information that can enlarge the file size, sometimes up to 90%.
So why do you need to Purge from time to time?
1. Reduced file size
2. Avoid potential problem data
3. Cleans out the layer list
4. Purges the following - Blocks, Dimstyles, Layers, Linetypes, Plotstyles, Shapes, TextStyles, Mlinestyles, Tablestyles, Regapps.
References in use are not removed.
Typing PURGE at the command Line, or File > Drawing Utilities > Purge.
To purge registered apps type -PURGE at the command line.
Command: -PURGE
Enter type of unused objects to purge
[Blocks/Dimstyles/.../Regapps/All]: R
Enter name(s) to purge : *
You can list registered applications in your drawing with a simple bit of AutoLISP. Repeatedly enter (tblnext "APPID") at the command line to step through the registered applications table. Or, if you have the Express Tool DBVIEW, you can view the Registered Applications Table in an AutoCAD Database Viewer dialog.