Ampeg B-1 and Ampeg 2-12 cabinet

[Vintage Gibson]

The reason for the front and rear facing speakers is to get direct and reflect off the back wall frequencies, which helps with harmonics and allows the length of the bass wave to develop.  Also, you could place the amp in front of you and have the rear facing speaker act as a monitor for you while you play.  I've built other cabinets using similar design and it works really well!  I get some really deep clean sounds, with the direct/reflect speaker placement and the phasing does pump more air that way, in spite of other opinions.