Approximately half of Foundation trainees choose to take a year out of training. There are any number of reasons that people make this decision from exciting travel plans to CV enhancing opportunities. Often it is just because, after five or six years of medical school and two years as a junior doctor, trainees feel like they need a break.
Despite the vast number of junior doctors taking an FY3 year, there are limited, structured opportunities and it is very much down to you to decide what you want to do. If improving your application is the main aim of your FY3 year, remember, that applications run across winter so anything you want to do to enhance your application will need to be done in the first half of your FY3 year.