There can be a couple reasons here. It can take a couple weeks for a cigar that has been mail ordered some time to acclimate to the specific RH you keep in your humidor. Letting them rest ensures you don't have any burn/taste issues due to uneven RH in the cigar. Also, depending on how fresh a cigar is it may go through a "sick" period where there is another fermentation cycle. Since the leaves are moistened for rolling, it can kick off another round of fermentation in the rolled cigar. Usually this is where you can get some ammonia smells while smoking. The biggest reason I tend to age my NCs is I feel I get a smoother smoking experience if it's been resting a 1 year+. You might be surprised how different a cigar is ROTT vs 2 years old vs 5 years old. For example, I love Aroma De Cuba Mi Amors, they are really good fresh, with about 5 years on them they are unreal to me. Some of the aging depends on the strength of the cigar as well. Some milder cigars may not hold up as well to age as the medium or full cigars do. As for how long cigars can last, with proper care they can last decades. If you have the room in your humidor, get a 5'er of a cigar you love and smoke one every six months and take notes on it.