You may not know this, but there are rules regarding how NFL players can select their jersey numbers. Matt Self shared to graphic to Fancred to more easily showcase the do's and don'ts of number selection.
So the question you might be asking is... why? Why go to such great lengths to regulate this aspect of the game? The simple answer: It makes the on-field officials' job significantly easier.
There are many, many rules in the NFL and most of them revolve around who can do what and when can they do it. Couple that with the fact that football is fast, and the time in between plays continues to shrink. The number system helps officials spot illegal formations (or confirm legal ones) with ease.