Queen rearing is an organised process where thought goes into selecting which colonies are used. It is not the same as producing new queens by using swarm cells from any colony that happens to be showing signs of swarming. When you select which colonies you want to raise queens from you are beginning the process of improving your stock. You can choose the colonies with the traits you like and avoid using those with traits that you do not like. In just a few seasons you can make a noticeable difference to your colonies.
Every year thousands of queens are imported into the UK to satisfy the demand from both commercial and hobby beekeepers. The reason for this is that countries in Europe (and beyond) can produce queens earlier in the season than we can in the UK and the spring is when the highest demand for queens occurs as beekeepers are sorting out the problems they find at the end of the winter and preparing for the forthcoming summer. However, with a little preparation and forward planning queens can be produced in the preceding season to accommodate demand the following spring. This means we can focus our attention on locally bred queens that are suited to our area and reduce the risk of importing pests and diseases. Rearing your own replacement queens is both interesting and fun but apart from that it makes sense financially. The small investment that may be required is quickly recouped once you have produced just a couple of new queens.