Do 300,000 km + vehicles have a purpose?

[Wild Cobra Z28]

Other than getting a trophy, is there something special about them?

Now I have figured out that there is a linear response of value to mileage. Any car you buy up to 100,000 km will lose 0.0005% value per km. At 100,000 km, it is half of the new value. Now I haven't seen any cars in the 100,000 to 300,000 km range, but if the trend continued, they would be zero credits at 200,000 km. Now at 300,000 km, the car is at new value, and then drops to 50% value at 400,000 km. I have only seen cars in the 0 to 100,000 km and 300,000 to 400,000 km range.

Do they rebuild to full or better value or something? I haven't experimented, and if there is anything special, I figure I might miss it anyway.

Is the trophy the only good of a 300,000 km+ car?

 

[Tornado]

Is the trophy the only good of a 300,000 km+ car?

Pretty much. That's also why the mileage scales so oddly as you pointed out; since it basically only shows up on those handful of cars just so you can get the trophy.

 

[watermelon punch]

If you're racing in certain types of multi-make race events regulated by PP, I could see some use to perhaps using high mileage cars... to fit certain cars into some regulations, without really heavily bearing down on the power limiter.

Do they rebuild to full or better value or something?

NO.
Full explanation here:
https://www.gtplanet.net/forum/showthread.php?t=182613
Short version here:
https://www.gtplanet.net/forum/showthread.php?p=8108559#post8108559

There are, however "car refresh" tickets, which I believe can restore any car to brand new 0 miles.