The original ball bearings that are used develop a flat spot over time and can cause uneven travel. By being adjustable these bushings eliminate the problem for the life of the machine.
Eliminate Lap slop and flutter. Ball bearings can't take the torque required by the lap and break requiring replacement. These taper roller bearings are setup the same way car wheel bearings are. They are made for the side loads developed by laps. They have to be factory installed to guarantee the top is perfectly aligned with your base.
The old nickel plated brass stop just doesn't have a solid enough stop. you can get false readings and when the plating gets worn giving you a very uncertain stop location. With the carbide stop you get the same stop every time and with other upgrades you can cut by the numbers for perfect stones.