Well no real reason other then space and cool factor. On my hotrods I have them on the firewall also along with vintage wrenches. You would see this on 20’s and 30’s race cars. So people seem to really like them and always ask.
"Because I can..." Eric Rothenhaus, Product Designer
After a google search, I didn't realize Spark Plug Caddys were used so much outside of Harley Davidsons