Miles and points are great. They allow us to accumulate and eventually redeem for travel such as airfare, hotels, car rentals, and other activities. There are many, many blogs out that cover the subject. Travelers can certainly use them to their advantage and save real money. Just be aware!
As great as they are, miles and points should be an afterthought when deciding to purchase one airfare over another. The primary goal of any airfare purchase should be price and schedule. If it happens to be part of a miles or points program you love that is a bonus. Loyalty should only apply to the notion of traveling in the cheapest, most comfortable fashion. Especially in economy class on long haul trips.
Many airline loyalty programs offer benefits to frequent flyers who achieve elite status. This includes: bonus miles, upgrades, free checked baggage, discounted fees, and so forth. The benefits are nice to have and do save some real money in some cases. Just be careful to not get too invested in one single program to the point where you pay more and take inconvenient flight routings or inferior service just for the sake of earning those particular miles.
Flying in an older, less comfortable plane for a long haul international flight versus taking a superior competitor flight is not ideal. If all variables are equal take the preferred airline and earn those miles. If the service, comfort, leg room, or any other important aspect of the flight is better on another airline consider it carefully. Most travelers would rather have a better in flight experience than a few extra miles in their account!