1. Paper is available in certain standard sizes. If your printed piece is over or under sized, you will need to use a bigger piece of paper and trim it down to size
2. If your color copies bleed, trimming is necessary
3. If you are printing many pieces on the same sheet, you need to trim: Example: multiple business cards on an 11x17"sheet

How do we trim?
There are different methods?

1. Static: using a paper cutter, industrial grade such as the one shown.
   
2. Dinaymic: trimming happens while the paper is moving in a machine. In between stations, circular knives are used, suction equipment removes the trimmed paper

Do you fold and then trim?

1. It usually works the other way. You trim your piece and then complete folding.
2. With other types of post processing (meaning processes after printing) such as embossing, scoring or die cutting, it might be better to fun the finishing steps before trimming.