There is some evidence that increasing the concentration oxygen in the air breathed can help with mild symptoms of altitude sickness.
Source here looks at long-term increases in oxygen concentration at high elevation and finds benefits--whether or not the same happens with short bursts from a canister is unclear. I have heard circumstantial evidence from friends in the skiing world that a hit from the oxygen canister works pretty well for mild exhaustion after a hike at 13,000 ft or so.
Since one of the main causes of altitude sickness is the reduced partial pressure of oxygen at elevation, increasing the ratio of oxygen to other gases in the air should help a bit, to an extent. Of course, when the pressure is really low, no matter how high the concentration or O2, the partial pressure will never be enough.
I'd also add that the source in Gabriel's answer is about hyperoxia at sea level, not how to mitigate hypoxyia at elevation. Then again, Gabriel is probably right that stores carry them because they sell, not because they necessarily work.
Edit re: how many breaths is the canister "useful" for: At sea level where atmospheric pressure is 760mmHg and O2 concentration is 21% the partial pressure is about 160mmHg. By 10,000 feet, that drops to about 110mmHg. To maintain sea-level partial pressure of O2, you'd need to be breathing about 30% O2 instead of 20%, which means 1.5 times as much.
So for each 0.5L breath of 21% oxygen air at 10,000ft, you'd need to inhale an additional 0.052L of 95% oxygen to get the same inhaled partial pressures as sea level. The 10L can would give you 192 hits of this volume. This fits nearly with the 200 breaths advertised!
Obviously, at higher elevations, you'd need more oxygen, but since these are mostly sold at ski resorts where 9,000-12,000ft is the norm, the 200 advertised servings per container seems reasonable.