The purpose of a buoyancy aid is to simply aid the wearers buoyancy in the water, unlike lifejackets which are life-saving devices. Lifejackets offer a higher standard of performance, turning the wearer on their back and fully supporting them with their face out of the water. Using a buoyancy aid assumes the user is able to help themselves either by swimming to safety or to tread water while assistance arrives.
You can read more about buoyancy aids here