Originally Posted by AGSF

CO2 cartridges are a great convenience if you get a flat while out on the road (a common thing).

Hand pumps are very different from floor pumps, and it can take many, many, many minutes to inflate a cycle tube (which goes in the tyre). Many (very strong people) are also not strong enough to get a hand pump to inflate the tube to any more than 60-70% of the tube's capacity, therefore making it likely that the rider is going to get another flat much sooner than if the tube was full.

Further, if you are biking in a hot location, or on a long ride, it can also be unhealthy/ dangerous to stop and spend a long period of time in the hot sun, and/or standing still. A CO2 cartridge lets you get on your way up to 10-15 minutes faster than if you just have a hand pump.

This is why the CO2 cartridge is such an essential part of a long-distance cyclist's tool kit.