why do you use "roll center ajuster"?

[Ozone]

RCAs do two things as mentioned above:

1) Roll Centre height.
Lowering the vehicle height lowers the Roll Centre, RCAs will lift the Roll Centre back to a height similar to previous. You can change it by using RCAs of different thicknesses

2) Bump steer.
Bump steer is represented as a curve on a graph of suspension stroke vs. toe angle. (Usually a vehicle is designed to toe-out in bump so in roll the front wheels steer away from the corner). As the steering arm is fixed to the lower link changing the vehicle height and/or installing RCAs only moves the starting position on the Bump Steer curve, it has negilable effect to the shape of the curve itself.
Note: this is only true of vehicles such as AE86, E30 BMWs and some others as the RCA is placed between the spindle and the steering arm. On most modern vehicles the hub/spindle and steering arm are the same part so would not be true for these vehicle.
If you want to change the bump steer curve you need something like these:
http://www.totalcontrolproducts.com/bumpsteer.html
These enable the steering height to be changed relative to the lower arm. This will change the Bump Steer graph curve.
However I would not encourage people to buy these!
You will need to accurately measure the change in toe as the wheels go from bump to rebound. Once you have this you need to know how much bump steer you want (zero bump steer is virtually impossible, bump toe-in will make the vehicle responce increase with roll which might be a little scary!) and make sure it is equal on both sides. This will require lots of time and/or money and you might end up making the vehicle "worse" than before. Toyota, although working to a different set of compromises to racers and drifters have years of research and development setting the bump steer so why change it thinking you know better when most of us don't have a clue. If you are building a racing car and the last 1/10 of a second is critical they might be a good investment but would be the last part you buy, for the rest of us it is overkill and something to get wrong.

In summary:
RCAs - Good
Height adjustable tie-rods - Bad, unless you really know what you are doing.