I've learnt it this way - and it makes sense.
SEP and MEP Aircraft as well as on the A320
Approach with Flaps 10 or max setting not being an approach setting. On intercept of 3 deg glideslope or visual descent - lower the gear then put flaps in approach or landing configuration.
Why?
1st It avoids nuisance warnings which some A/C have. The Pipers do. When putting flaps in an approach configuration with the gear up. A320 does too.
2nd It adds the drag needed for a steady descent with a reasonable power setting thus avoiding to get too fast when approaching with a low flap setting. Especially in A/C with a tendency to pick up speed when on the glide slope. At what altitude is of no importance since the begin of the final approach in descent is master. Use the gear to slow down also helps not to mess with pitch when using speed brakes.
3rd As stated before it gives you plenty of time to check proper operation.