For a single day ride, I carry the following:

One spare tube
One patch kit
One mini pump
One Co2 inflator with one cartridge

If I have a flat, I always use the pump to air up the tire to about 30 PSI or so, then use CO2 to fill the rest of the way. I view the CO2 only as a way to get full pressure without excessive pumping on a small pump.

If you carry a pump, you can always fill a tube. You can also repeatedly add air to a slow leak and might get home without changing tubes.

If you only carry CO2, how would you put a little air in a tube to find a leak? You might have more flats than spare tubes.