When working on tub or shower faucets, most of the time you remove the cartridge and have to either look for it in your work van or drive somewhere to get the cartridge. Since there are an endless number of faucets, you have to take the cartridge with you to match it up to the new one to ensure a correct match.
Meanwhile, the water pours out the open shower valve and runs behind the customer’s wall. This makes a huge mess, an even bigger mess if the tub or shower is on the second floor! There’s an easy solution to this.