OK a-arm 101...
extended a-arms can do many things,
1. widen the bike, wider can be more stable
2. provide servicable ball joints as the stock ones cannot be replaced
3. can increase wheel travel, how much depends on the quality/design of the a-arms. A ball joint only has so much articulation or angle that it can move., a longer arm provides more travel for the same ball joint angle. Also if they are well designed, the angle that the balljoint mount to the a-arms will be maximized to provide even more tavel, while the shock mount will be slightly relocated to utilized the available travel
4. +2,+3 etc. is how much wider the arm is than stock
5. The second number +0, +1 etc indicates how much further foreward then stock the ball joints are located. Yes this increases the wheelbase, but more importantly it changes the weight bias/ centeral mass axis. picture a teeter totter ___.___ the dot is the axis. when you lenghten the swingarm to stop the front end from comming up, that moves the mass foreward making it harder to lift the front end by placing more weight on the front tires. When you go +1 it moves the mass rearward lightening the front wheels.
6. allows the camber to be adjusted (tilting the tire) this can allow better handeling.. by tilting the tire a little , when you are turning really hard, it will give the tire a better angle to the ground as the bike leans
7. Some a-arms have adjustable caster. this is a front to back differance between the upper and lower balljoint. adjusted one way, the bike will turn better but will not be as stable in a straight line, adjusted the other, the bike will be more stable, especially at higher speeds, but will not turn as easily.