Direct sunlight is a very hard light - that means the edges of the shadows are sharp and that the shadows are very dark - people tend to look bad in this light.
A big reflector near the model reflect soft light (because it's big and close) even if the light hitting it is hard sunlight.
So, if you position your model so that most of the are facing the camera are lit by direct sunlight with a reflector on the other side you have a hard key light with a soft fill.
But if you position the model so that most of the area facing the camera is in shadow and then add a reflector you suddenly have a soft key light - much better looking.
At the extreme you can position your model with his/her back to the sun (obviously this doesn't work mid-day when the sun is directly above) so what you get is basically a silhouette and then add a reflector in front to be the only light hitting the model - now you have a soft light portrait with the hard direct sunlight working as a heir light or a kicker.
You can always control the direction of light (relative to the model and camera) by moving the model and or camera, you can always change the light quality and color with a reflector or random objects you find in the shooting location - difficult ambient light just means you have to be more imaginative and work harder.