they allow for the use of 2 brake cables if you need to configure it as such, i.e., if you travel alot, have an S&S coupled frame, etc.

BUT: stock shimano cable ends are too fat to fit in the barrel, you have to shave/file fractions of a mm off of it in order to slide all the way down inside. which means if you're in timbuktoo, in addition to carrying an extra cable, you need a flat metal file (or your wife's nail emery board!)

i got 'em originally because i didn't know they made single brake (and derailleur) cables in tandem length. i recently discovered this fascinating thing called the internet, which informed me otherwise....