While recreational scuba diving has a significant lower rate of accidents than other sports, emergencies happen and you should be prepared. But maybe you think that you will never run out of air because you have a gauge and you check it frequently...well, that is not completely true, because in the underwater world you have an infinite number of factors that can affect your diving experience.
Even the most experienced diver has faced in his life some kind of emergency or air related incident. In case you are wondering, how somebody can reach the point where he or she doesn’t have air?, some of the main reasons why people run out of air while scuba diving are generally due to failure in equipment (o-ring, gauge, tank, regulator, octopus, etc.) or external factors like diving in cold water, against or in a current, being trapped in a fishing net or under a rock.
In any case, you will need an additional or alternate source of air, or that could become your last diving experience. There are many options and techniques (like buddy breathing, octopus, pony bottles, etc) to overcome the problem of air shortage, but I will highly recommend you to consider buying a Spare Air system. I am not affiliated with any manufacturer, but it simply makes sense!
When you are shopping around for these kinds of scuba diving systems the key question is: What is your life worth?
Air is precious, especially when you are at 70 or 80 ft.
