1/4" In-Line Ball Valve
The 1/4" in-line Ball Valve can be used to stop water from flowing through 1/4" tubing. It can be installed before the RO system for easier filter replacements, or between the bladder tank and the DI section. It use useful for water storage with a float valve: Once the water has collected, close the in-line ball valve to shut off the RO/DI system until you need to replenish the container.
I'd suggest getting a few of these, they are super handy.