The holes change what is called the Ackermann angle.
When going round a corner the outside wheel is further from the centre of the circle than the inner wheel, so the inner and outer wheels have to point at different angles.
Clearly since the wheels are straight and the corner is curved, at any moment the wheels are not pointing 'round' the curve but away from the curve.
The holes in the steering arms, with the holes in the steering column, allow you to set up the steering so that the wheels either point inside the circle or outside the circle**. there are advantages to either situation that will change with each track, each driver, tyre softness and flexibility.
** This results in changes in toe in and toe out, which also have an effect on how the kart handles
the basic situation is described fairly clearly here http://www.rctek.com/technical/handling/toe_angle_effect_on_ackerman_steering_principle.html
(yes, I know it's for RC cars but it's the principle)
Broadly, toe out in the corner improves turn in, but at the cost of stability in the corner, but this will depend to some degree how you have set the Toe in/ toe out in the first place.
Like so many things in karting, there isn't a simple rule of thumb that works in any situation... for example if you set the kart up with slight toe out, you may not wish to add extra toe out in the corner, if you set the kart up with toe in, then you may wish to set up additional toe out in the corners.
In addition, you will need to test whether you lose more in long, sweeping corners as a result of aggressive ackermann which may help in tight corners and so on.
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