Using this parameter for describing physics of parts of an addon slows down vehicle as a whole. Screenshots lower are a comparison between the speed of vehicles without the addons with LinearDamping on them as opposed to the top speed with the addons that have this parameter.

The parameter is also present in the configs for roofracks of Kenworth 963 and Kenworth W990:

And these are only the recent ones, the full list stretches all the way to release day trucks.