Why do you use slotted armatures in dc machines?

Those slots, or areas where metal was removed serve to balance the motor's armature. This is similar to the concept of "balancing" a wheel on a car.

Metal material is 'shaved' or notched ( removed) from the side of the armature which is too heavy, ( and causes a ' wobble' as the RPM's are increased.)

However, On a car, wheel weights are added to the OPPOSITE side of the wheel's heaviest spot of imbalance, until the wheel can be spun at a high speed, with no wobble.

Answer

The slots carry the armature conductors, while minimising the air gap between the rotor and and the stator (to minimise the reluctance of the magnetic circuit)!

They have nothing to do with 'balancing' the armature.