Clay Christensen asks whether understanding our customers actually helps us innovate? Knowing about the characteristics of people may only help you get to a correlation between people and the things they buy. Whereas understanding the job they need to do gets you closer to the causation of why people buy something or not.
Understanding the job people need to do (e.g. furnish their home in a day, keep busy on a dull commute) helps companies understand which dimensions of their product/service they should be innovating on. It’s pointless spending time innovating on a dimension that makes no difference to whether people actually buy or not.
This understanding also helps you know what your competitive set really is (it might not just be made up of direct competitors with the same product as yours).
Clay illustrates this in characteristic dry fashion in the first 15 mins of this Boxworks keynote (whole thing worth a watch if you have time).