Power banks are designed to keep smart phones going when their batteries are just about flat. People buy a power bank for smartphones to make sure they can keep their phones going until the battery can be charged again. These power banks are small and can be taken anywhere.
Some banks are the same size as a credit or debit card, and provide a power boost for smart phones for one or two hours. That should be okay providing you can reach a charger relatively quickly. The size and slim shape of these banks are ideal to have in a wallet or a purse for emergency use when the smart phones are about to have no power left at all.
Alternatively there is a larger type of power bank that has bigger batteries. These banks are still pocket sized but their extra size means that keep smart phones charged up for longer. For people that constantly use their phones these power banks are worth buying and using.