Rectangular reflectors perform better on people than the round reflectors do.
One of the big problems with only using available light and reflectors is you are so limited as to set ups.
Overhead light sources tend to cause 'racoon eye', and dappled light is a nightmare.
All light is 'natural' light, but comes in different flavors.
Sunlight has color temperature variations throughout the day. The color temperature is even different depending on the direction the camera is facing at any given time of day and time of year.
In other words, the sunlight available now is very different than the sunlight that will be available in 6 months because here in the northern hemisphere it is so much lower in the southern sky. In July, the Sun will be higher in the sky and it's also likely the amount of particulate matter in the air will be greater too (air quality).
By using strobed lighting (flash) all those variables are of little concern to the photographer, becuase the photographer has way much more control of the light.