Many guitars with bolt on necks have no neck angle. This means that when the neck relief is set to zero, the fretboard surface is parallel to the body.
Some guitars have some degree of neck angle, usually 2-6 degrees. Why?
The first reason is simply age. Guitars are usually made of different types of wood that were cut and dried at different times. As things age, things move. What was once a perfectly set up instrument may have changed with time. No matter the quality of the original build.
Because of the amount of neck relief needed to allow the strings to move freely with no buzz (if this is your preferred setup) and the height of some bridges, some neck angle may be needed to keep action low. Some flat guitars with tall bridges may need some neck angle, for instance.
Fretboard radius also plays into the measurement. A small radius such as 7.25″ is taller in the middle and shallower on the sides than a 12″. This makes the relationship from the fretboard surface to the bridge plane (the surface the bridge is mounted to) different. This requires the string height to be different. Sometimes, neck angle is needed to achieve a more playable instrument with lower action.
On a bolt on guitar that was designed with no neck angle, shims are the easiest way to achieve this. Using a matchbook cover or a piece of cereal box has long been the common way to shim. Remember, you are only looking to raise the heel of the neck a few thousandths of an inch.
Needing a shim is usually not in any way related to the build quality. Even Leo Fender knew that he would have to shim necks for certain setups. Fender even introduced the “microtilt” adjustment as a fancy way of “shimming” a bolt on neck.
Not everyone needs to shim their bolt on neck. It is all up to preferred setup, but it is worth playing with if you are trying to achieve low action with certain combinations of fretboard radius and bridges.