I like to carry as little repair tools and supplies as possible when commuting, so its covenient to convert all of my schrader rims to fit all presta pumps and tubes instead of a mix of each kind.
I was trying to use tubes with schrader valves on my recumbent trike. The front rims are 406 (20) size and my tubes bunched badly inside the tire. Expected lifespan for the tube was short. Presta valve tubes fit my 20 rims much better. Since the tube is thin rubber and air pressure is 90-100 psi, the tube might blow out around the valve stem. The stem saver prevents this. It also holds the Presta valve stem straight during installation so the rubber on the stem can't be cut by the edge of the hole in the rim. This reduces chance of leaks, increases reliability.
These tiny little bits allow you to run the far superior presta valve on an old (or cheap) wheelset that is drilled for Schrader valves. Other shops will charge you twice the price!
I really like this item. By converting my shrader rims to safely accept presta tubes I can standardize and consoladate my inventory of spares tubes. I like these better than the rubber gromet style converter.