I don't really feel like this is better than Zen Monkey's answer, but worth a shot. Is it
a bed?
...It was pretty darn flat
Beds are pretty flat
Some people may even call it a mat
Some people sleep on bamboo or foam mats, that's pretty much what college dorm mattresses are
Supporting an integral tool to some, yet a dying tool to many
Not really sure about the "tool" part, but many people die in bed.
At least compatibly, it supports any!
Anyone can lie on a bed
Making it easier to move, even to be precise
Perhaps a hospital gurney? Makes it easier to move someone who is incapacitated
To its beneficiary's homograph, you probably won't be nice
No idea about this one either
It's used at home, in the office, but rarely on the road
People generally sleep at home. Office is a bit of a stretch, but maybe a doctor's office? In vehicles there are not usually beds, unless you're in an RV
Some even aren't flat at front, to support the user's load
Some beds like Sleep Number beds can be tilted at the upper body for greater comfort. Also hospital beds can usually be raised.
But the largest portion is, or else there'd be no point
The majority of the bed is still flat and horizontal
One last clue might be pain, in a specific joint
Not sure about "specific", but joint pain often makes people stay in bed longer
