Queen cells are associated with grafting queens and queen cell raising.
If you are looking to rear your own queens you use queen cells to do so.
They are vital in helping raise the queen cell.

Used best when attach to a frame as a cell bar frame.
Graft 12-24 hour aged larvae into the queen cell cup.
(if you can graft royal jelly with the larvae on top it is recommended if possible)
Once the frame is ready, place into the designated hive and continue the rest of the process of grafting accordingly.