Every photog knows the about the 'golden hour' for the gorgeous quality of light that is produced just after sunrise and just before sunset each day, but what most don't do (including me 'til now) is get enough fill on the subject to balance out the extreme side lighting that a low sun will create. I've usually been using a flash for this work but that can cause some variations in color temperature across the frame unless you gel your flash. A reflector is a easy (and cheap) way of doing roughly the same thing.
Above is a picture of Ryan where I placed the 5pm sun behind him to act as a hair light, then used the reflector to bounce the sun and create the main light for his face. I placed him in the edge of a shadow created by a near by house to throw the background tone into a lower exposure value and let his face stand out against it but kept the foreground lit to keep the interesting shape of the weeds. And, my favorite part of a portrait, catch lights in the eyes. With the sun behind him the reflector is the only way short of a fill flash to get those eyes to sparkle.
Contrast that with the following with out the reflector.
While I still get a nice side light, but the catch lights are gone, his face is flat without any shadows, his face falls into the same exposure levels as the background and even if I bump the exposure a bit, the side light would probably clip and his face would still blend with the background.
I'm pretty convinced that a reflector is a 'must have' item for any portrait photog - they don't break the bank either, but if you're really cheap like I am you can use a Car Sunshade, I got mine at target for $5.99 and its even silver on one side and gold on the other - almost like it was designed to double as a photo reflector!