Save a line item instead of retyping it
A saved item keeps the name, the description, and the rate. The next invoice can use it again.
What is stored
Name, description, and rate. A module, a day of design, a carton. You are not building a warehouse. You are saving the line you bill more than once.
The invoice can still change
Picking a saved item fills the line. You can change the quantity or the rate on that invoice. The saved item stays as it was, until you edit it.