Back in 2011, anything over 10 yards of a throw was pretty much a contested catch. Tommy Rees was fortunate that he had receivers the likes of Michael Floyd and (in 2013) TJ Jones, who could fight for those catches and have a reasonable chance of winning them. Without the vertical game, and with no QB running threat, half of the offensive playbook was all but crippled in 2011 and 2013.
The running joke (sad, but true) was that to stop the ND offense in 2013, all you had to do was rush three, and drop 8 into a short zone coverage. It's no secret that Brian Kelly didn't trust the running game in 2013, even though we were averaging about 5 yards a carry (if you don't include Rees' sack yardage), which further crippled the offense.
While Ian Book will never be known for having a strong arm, and could arguably be classified as on the lower end of average arm strength, it's still a significant improvement.
Ian Book can make those 15 to 30 yard throws with some decent zip on them, which opens up a lot more of the vertical game than we ever had when Tommy Rees was at the helm. Those throws by Book are much less likely to become contested catches compared to Rees.