The purpose of a bike pump
Bike pumps are an essential piece of kit for both casual and serious cyclists alike. Even if you're lucky enough to not get a puncture, tyres slowly lose pressure over time, and different situations may require different amounts of pressure.
Bike pumps have two purposes: to keep your tyres at their recommended pressure level and to fix flats. Keeping your bike's tyres at their recommended pressure level helps to reduce rolling resistance and reduces your risk of punctures, whereas having too low a pressure can start to damage the rims.
If you get a flat, a pump won't be the only thing you need; don't forget to take a puncture repair kit out on the road with you too.