Guys those are setting numbers not cm or mm from the ground. a 6 front setting is not the same 6 rear rideheight setting. The car already comes at a rake at 6 6 ride height.
By lowering the rear, you have less rake, which turns into less drag but less rear downforce. In turn it does also affect the front and have less front downforce (not much tho).
It does help the car in slow corners as the centre of mass will be closer to the ground, so actually in slower corners you will feel better traction and more grip in general.
Setup is all about compromise as you all well know, and also the way we setup the cars in this game is different. If in iRacing, AC or rFactor you can actually set the distance of the front and rear of the car to the ground, in F1 you cannot see that value.
Out of the top of my mind, I think you have to set something like 10 1 to have a negative rake, but I can guarantee the car will be pretty bad to drive.