Purge
is an essential command to keep drawings organized,
reduce file sizes, and maintain performance. It is useful
after copying and pasting other objects from other drawings, since this operation can import many unseen elements to a drawing.
Purge the drawing to get rid of all unwanted and unused layers, line types,
font instructions, etc...All these things take up space, and also
can slow down the performance.
Autodesk article on purging unused layers.
