Does a beekeeper have only a few minutes per hive to check for swarm cells? Best quick check . . .
Swarm cells are planned by the workers and therefore are placed close to the center of the brood nest - the safest, warmest, most protected spot where the cells will receive the most attention. When the broodnest spans more than one box, the bottoms of the frames of the top brood box are prime locations for those royal cells. If there is no time to properly separate brood boxes for a full hive inspection, or too many hives to check them all thoroughly, often a beekeeper tilts the top brood box enough to see its underside. Queen cells hanging from the bottoms of the top box frames is a colony's preferred location to raise queens.
Also, checking for swarm cells by tilting a box before removing it and sliding or setting it to the side, allows a beekeeper to avoid injury to those cells. After all, swarm cells can be used in making splits/nucs for sale or apiary expansion.