You could use a shim for a number of reasons. You could move the neck slightly closer to the player, or slightly further away. You might want to raise a low bridge, or lower a high bridge. Your pickups might not raise high enough, lots of reasons. I recently built several J-style guitars. One seemed just right as it was. The other, pictured below, had the Mastery bridge practically sitting on the pickguard. The 6th string was making contact with one of the trem screws. I made a 1 degree full pocket shim. You can see the results of that in the pics below. 1 degree is too much. I have since made a 1/2 degree full pocket shim. That's a keeper. A little shim angle goes a long way due to the geometry.
One degree full pocket shim:
Look how high the bridge is now. The pickups cannot be raised high enough for good adjustment.
For those that want to save time and not experiment with various shim angles, there is a formula for determining which angle to use: (XZ)/W
It's probably been posted here before, but here's a link if you are interested:http://fingerlakesguitarrepair.com/fender-neck-shim-bolt-on-neck-reset/