Caffeine is a stimulant which makes you feel less tired and, for athletes, delays exhaustion. So if the alternative is being asleep, caffeine cetainly helps.
The other issue is that the regular use of caffeine leads, as with other psychoactive substances, to dependency ( _tolerance_ or _addiction_ are other words used). In such cases, lack of caffeine leads to withdrawal symptoms such as drowsiness and irratability. For dependent individuals, a cup of coffee will remove these symptoms in the short term, aiding concentration, though it is less clear whether this is an improvement on never having taken caffeine.