I'm sorry if this sounds like a stupid question, but what's the reason for setting par based on a player's skill level? Shouldn't par be based on the course alone, and not whether I'm a red or white or blue or gold level player?
It's not a stupid question at all. That school of thought is directly validated by one of the definitions of par in the rules: "The score an expert disc golfer would be expected to make on a given hole."
As Chuck pointed out, the par you get by using only a top player as the Expert is less workable than if you have different pars for different types of players.
Par is more workable if the Expert is someone who is really good in the group the player is competing in. That way, if they player is way over par, everyone knows that player is way behind the leader.
The groups could be defined by Pro/Am, Male/Female, Age, Weight Classes, Years of Experience, or any way to group similar players together. But, we don't have a lot of data for most of those groupings. So, for now, we use Ratings.
It would be more difficult for us mortals to keep score in our heads if we have to keep adding 2 or 3 to our "over" total.
Also, different Pars (and/or different tees) allow for finer gradations between different holes. If we used gold par, every hole up to about 650 feet would be par 3. For green players, they would have no way to tell whether they should expect to get a 3, 4, 5, or 6. About the 5th throw on a hole, they'll get frustrated unless they are still on track to get par.
Also, it would be more difficult to tell whether a player is doing well or poorly in a tournament. Is "35 over after one round" good for an Advanced Grandmaster Female? Uh....
Using only one Par would be very similar to using only one set of tees for everyone. Yes, one tee length and one par would be the purest form of competition. It would quickly establish who is best. But that would be boring and nobody but the best players would have any incentive to play again. Different divisions, tee lengths, and pars allow all players to get the same kind of experience as the best players.