The big advantage of using panniers is that you can carry more without having to put anything in a bag on your back. This makes them very popular for touring and commuting purposes. So, if you want to take a picnic, or camping gear or even just a bike pump and snack food with you on your ride, you can do so without being weighed down by it.
Panniers can be fitted to the front or back of your bike. It’s normally best to get them in pairs, so that weight is equally distributed and your balance is not affected. Bigger panniers can carry more, but will also widen your profile and create more drag.