My sight is pretty poor and this certainly influences the games I buy. For example, I don’t even try to play Football Manager anymore because of the eye strain.
krygerleeu, via Eurogamer
As well as being an important way to cater for people who have different screen sizes, manually resizable interfaces are also useful for people who either have difficulty seeing small elements or who don’t have the fine motor ability required to tap/click on them.
Although ensuring a good default size should always be the first port of call, some people prefer smaller UI elements that obscure less of the gameplay, so allowing it to be configurable means that both ends of the scale can be met.