A battery pack is an incredibly practical thing. As an occasional "energy bar" it supplies your mobile phone, sat nav et al with an extra portion of current when you are on the road, so you should never be without it when on a longer tour. In an emergency the energy from a power bank is even enough to jump start your bike. By the way: it doesn't really matter whether you refer to it as a power bank or a battery pack. The storage capacity and the connection options are more important.