That is popular in the Northeast esp. in heated garages so when the snow melts off your car it has someplace to go. Also if you have a monolithic slab and build with wood on grade it can help prevent wood rot.
If you have a frost wall or block wall on slab before lumber this is not a problem. With the snow melt arguement: also in there will be going salt, oil, antifreeze whatever else leaks in your car or "accidentally" ends up in there. A lot of shade tree mechanics like this.
For this reason I believe technically a permit is required in most places and probably it should run through a seperator filter system blah blah... I dont want to go postal green
on you but you get the idea. If his reasoning interests you check with the local CEO first.
IMHO, I pitch the slab instead. Then I do radiant heat in the slab... the snow turns into water vapor before it gets to the door seal anyway. Plus I "sweep off the car first" ... now there's an idea... LOL Anyway, I like around 1.3% grade, I've used this in 21' deep to 36' deep garages.
Let us know what you decide and why and how happy you are with YOUR choice.