B is the fifth of E and the major third of G, it’s also the 9th of A, the 4th of F#, minor third of G#, minor seventh of C#, major seventh of C and the 6th of D. So there are a lot of open chords you can play with the open B which is pretty convenient. If you would make the open B a C(so everything works in 4ths again). The C is the 5th of F, minor third of A, the 4th of G, major third of G#, the 6th of D#, the 9th of Bb, the minor seventh of B, major seventh of Db. So, when it’s tuned as a B you have more options for open chords than when it’s tuned in C for example.