A few reasons
1: While it's acceptable to use the screws to earth the back box AIUI it's not acceptable to use them to earth the socket and stuff plugged into it. So if you have a system which relies on the conduit for earth then you will need a flylead.
2: Plastic lightswitches don't generally have earth terminals or earthed screwholes. Some other accessories may be similar.
3: Just because BS7671 doesn't require earth flyleads doesn't mean they are not a good idea. Especially if installing in adverse conditions.
1: While it's acceptable to use the screws to earth the back box AIUI it's not acceptable to use them to earth the socket and stuff plugged into it. So if you have a system which relies on the conduit for earth then you will need a flylead.
2: Plastic lightswitches don't generally have earth terminals or earthed screwholes. Some other accessories may be similar.
3: Just because BS7671 doesn't require earth flyleads doesn't mean they are not a good idea. Especially if installing in adverse conditions.