The 'philly shell' is quite a complex style with so many subtleties and variations that it's very hard to break down. It is often refered to as the 'shoulder roll defense' because the shoulder roll is one of the defensive skills that is incorporated by most boxers that use this style.
Since most of the interest in this style is from a defensive stand point, I will start with the defensive benefits of using the philly shell. The first defensive skill I'll talk about is the shoulder roll. The shoulder roll is very effective for defending against the orthodox right hand, specifically the straight right. Not only does it ensure that the chin is well protected from the shot, but if executed effectively, it sets up clean counterpunching opportunities.
3:10 (3:17 for slow motion) shows perfect execution of the shoulder roll by Mayweather. Not only does he protect his chin and deflect the shot away with the shoulder, he comes back with a nice short shot of his own, which Mosley is wide open for. With the shoulder roll, the orthodox fighter will turn his shoulders clockwise as the punch is anticipated and as this is done, it also takes the right hand out of the opponent's sight, which makes it a lot harder for them to defend against a counter that they've already been opened up for.
Further examples of the shoulder roll - counter right hand being executed:
Another defensive benefit of the philly shell is that when a fighter is backed up against the ropes, because of the hand position, they don't restrict their own vision and this allows them to parry, slip, roll and counter against punches.
A few examples of this:
Due to the hands are held in the philly shell, so many defensive moves can be executed with minimal effort and energy expendature.
At around 1:45 Floyd Mayweather demonstrates how with very, very small movements shots can be blocked and present huge counter opportunities.
Since there is so much involved in this style, I'll make another post with the offensive benefits and if anybody wants me to try and explain further any defensive specifics, just ask. I'm not great at writing out long break downs