There's no doubt that solid depth is necessary to avoid a big drop off in performance as games and the season wear on. You've got to account for fatigue and injuries, and without quality depth, it's impossible to sustain high-level play. Ole Miss was a prime example of this on Monday, as the Rebels lost a couple of key starters and really wore down, leading to an absolute throttling at the hands of FSU in the second half.
So, building good depth at every position is key in that regard, However, Kirby and the staff have, in my opinion, a paramount reason for building up the roster. Capable and quality depth encourages top effort through competition.
At this point, most avid Dawg fans are aware of the debacle that was the 2013 recruiting class. They are further aware that, from time to time, Georgia would essentially whiff on top-flight talent at various positions for a year or two, causing gaping holes in depth. This was no more apparent than at the OL, WR, and QB positions.
What ultimately happens is that a player will settle into a starting role because there's nobody there to push him, save a walk-on or low-end 3-star or 2-star recruit. This is not to call anyone out or suggest that they're not giving good effort, but it's often human nature to just give "enough" effort.
If a 5-star recruit has no quality depth behind him, then giving 80% is going to trump a walk-on's 100% just about every time. With depth at a position, now you have two 4-star guys giving a 100%, which will beat out a 5-star's 80%. At that point, you're either going to get 95-100% out of the 5-star, or you'll be better off with the 4-star, with that 5-star backing him up. Whichever the case, you're now in much better shape. Never mind being able to account for injuries or "busts" in recruiting.
Obviously, nobody compares to Alabama right now in terms of depth across the board. The Tide have a 2, or even 3-deep roster of former 4 and 5-star recruits who would be starting almost everywhere else. They also are able to become more seasoned, bigger, stronger, faster, etc. while they wait their turn, instead of being forced into action based upon talent and upside alone.
The bottom line is this: You better dedicate yourself fully to becoming better at every facet of your position, because now there's someone just as talented as you ready to take your spot if you don't.