Roster depth is very important to a team for various reasons such as having players who can step in for injured players (important given the nature of football), allowing players to learn the system/schemes before playing (despite what many think of football it is complex and being able to "reload" rather than "rebuild" is key to the perennial powerhouses), giving players time to develop and grow before seeing game time (most are a mere 18 years old when starting college), being able to field scout teams for the starters to practice against, etc.
USC football is currently facing sanctions and lost 10 football scholarships per year for three years. Read any sports analyses on the sanctions and they'll tell you that the scholarship loss hurts USC not because so much because top recruits will be making something of themselves elsewhere but because it hurts USC's roster depth.