They also work as a choke, the benefits of which people are just finding out.
I used to have an AMP tank (that I sold after receiving my first two Divers) and I held on to the 510 atomizer tube extender. I used to use cartomizers with 19mm XL tanks (that went well with my Provari) but I now use 22mm standard tanks with my Divers and mechanical mods. I really like the smaller form factor of the standard tank now, but I think I'll have to break out an XL tube and try the choke.