You have to Size the Brass before you Trim it.
Measure a piece before you size it and then afterwards.
You will notice that it increased in length. (In most cases)
If it is still in Spec, you do not have to trim it if you don't want to.
A lot of people trim to a set length each time to keep everything consistent.
Edited by dredd (12/08/17 12:50 PM)