Why would you need a new metal detector shaft?
There are many reasons a Metal Detectorists may need to purchase a new shaft. The obvious reasons is to replace a broken shaft. It will most likely happen to any serious metal detectorist at one point in their career.
There are however many other reasons. One might be conditions. For instance, there are shafts made from all sorts of different materials. Many stock models come with a molded, high density plastic shaft. Most usually have a metal upper and a plastic lower. There are companies that have developed some awesome carbon fiber metal detector shafts that can help reduce the weight of a detector noticeably.
One unique reason someone may want a different metal detector shaft is if they have multiple coils. Say you have an Equinox 800 with a 6" coil and another 11" coil. Instead of having to take the coil hardware off, remove the coil, replace it with a new one, and re-mount the new coil and hardware, you could already have your alternate coil mounted to another shaft ready to quickly and easily be connected to the detector.
Having a variety of coils is beneficial depending on where you are metal detecting and what you are looking for. Since this may change from detecting site to the next, it is wise to have your arsenal of metal detecting equipment complete. Being able to quickly swap out one set up for another just gives you more time to get out and dig some treasure!