When machinists mount work pieces for grinding they often use magnetic chucks and fixtures are set on those chucks. The thicker the fixture, the more the magnetism is attenuated. Sometimes that is a good thing, because sometimes you do not want the work piece to be held too tightly. Other times, however, you want the magnetic holding force to be strong. In that case fixtures are slotted because this improves the magnetic flow. Here is an example of a slotted v-block:
This kind of v-block will transfer magnetic force from the chuck much more powerfully than an unslotted v-block will. Machinists have learned this from experience, but what is the theoretical explanation? Why would a block with slots in it transfer magnetic force better?