Camber plates off an obvious advantage of gaining additional negative camber over what can be gained just from the oe eccentric bolts. This is still the biggest advantage of camber plates, but beyond this obvious advantage there are several others folks might not be aware of.
First by manipulating the orientation of the plates you can gain addidtional positve caster (in addition to more negative camber)- the amount varies by plate design and the extra caster means sacraficing some neg camber, but often yields more than enough neg camber for a lot of folks.
Second there is a fair amount of deflection in the oe strut top- Whiteline measured as much as 5mm- this was in hard cornering- not going off a jump A Group N mount will reduce this deflection but not eliminate it, a metal camber plate virtually eliminates it.
Thirdly by moving the strut tops inward (this is how camber plates increase neg camber) you improve the roll center. If you've read much about lowering you know that it negatively impacts the roll center (it's relation w/ the center of gravity specifically)- camber plates help improve this relationship when lowering (bit doesn't fully negate depending on the amount of lowering)
this has some pretty neat modeling diagrams showing the effect of moving the strut top inboard- courtesy of zzyzx motorsports
Lastly if you look closely at the diagram you'll see a change in scrub radius as well- when we change the offset of or wheels- almost always lower- we impact scrub radius. By moving the strut tops inboard our scrub radius (w/ our lower offset wheels) is very near what it originally was from the factory
I'm certainly not advocating camber plates for everyone, quite frankly they aren't needed or justified for most applications, but wanted to point there are more advantages than meets the eye.