There isn't a single reason why you should know or care how to read a nutrition label. If you're the type of person who shops and carefully considers the 'nutrition facts' on the food you buy, you're probably the type of person who is eating the wrong food. 

Real food does not have a label, and nutritional requirements are highly personalized. The best anybody can do - short of individualized food testing - is to give you a template, and the only template we've discovered so far is something akin to paleo - which simply means REAL FOOD. 

It's true that most paleo advocates will suggest you limit or avoid grains, legumes, and dairy, but as long as your template keeps you miles away from the jungles of misinformation promulgated by the corporatist agro-gov harbingers of unhealth (label-makers), you'll still be headed in the right direction.

If you weren't inspired to click on the previous link, here's a template you can use to help self-author and correct your eating habits.