The "medal" ratings are meant to be a measure of a driver's abilities, based on performances and results across the past few seasons. The boring reading can be found
here if you need it. They are updated yearly, and there are stipulations for age that have a driver automatically move down a rating.
As it applies to sportscars, many series use driver ratings to place restrictions on who can race where and with whom. This is typically done to ensure that gentleman drivers - who do not have as much time to commit to racing, yet bring significant amounts of funding to privateer teams - have time in the car. Driver ratings are more generally used to limit how long a driver can spend in the car, for similar reasons. Restrictions based on driver rating are a way of keeping the playing field level; to quote the adage, "You're only as fast as your slowest driver."
The key contention is between silver and gold. This is seen as the line between gentleman drivers and professional drivers. However, there have been many cases of "super silvers," silver-rated drivers who were not rated higher because of a lack of results, but who had the pace and consistency of gold or even platinum drivers. This has been exacerbated recently by the increasing popularity of sportscars, particularly prototypes, as an alternative career to high-level single-seater racing, bringing many young soon-to-be professional drivers with it. Many series and classes mandate at least one silver or bronze driver in a lineup, so the quicker this driver was, the better for the team.
This isn't a full explanation, but it should give the gist of the issue. The recent mass change to the categorisation (and subsequent
mass of successful appeals) shows how important this is for teams and drivers.