Some 3rd party blueprints and some hardware based projects might expect the same bone rolls on characters as the UE4 Mannequin. I know atleast a couple of cases where this was needed by Uefy Script customers. Any project that is based on shared animations built on the mannequin will need this too.
For example in https://youtu.be/4Dh5k4ATHYU ALS system is built on the mannequin. With set original bone rolls new characters can be dropped into it very easily. While I can not comment on this system specifically but generally trying to update a system of blueprints like that can be very difficult. So compatible characters with correct bone rolls are needed.
If you are just working on your own project it is probably not necessary but would be very desirable if making content for the unreal marketplace. While Epic has not specifically stated so, not having the correct bone orientations will cause problems for some customers for the above reasons.