You'd be surprised how often you come across a situation where not knowing standard photo sizes causes problems - whether its trying to squeeze a photo into a frame, or making plans for some wall layouts to display your favourite snaps. Knowing those standard sizes makes all the difference - it makes it way easier to get the right frames, get the right sizes and layout, and save you time and cash in the long run. As Visuals Clipping suggests - being on top of standard photo dimensions makes it easier to create a display that looks tidy and professional, whether your decorating a room or just trying to show off some happy memories.