If you use sealant in your tubes, a removable core makes it easier to inject the sealant, and if/when the sealant clogs the valve, you simply remove it and soak in ammonia to dissolve the sealant. I find it handy to have a few core available, so I can swap a clean one in while letting the clogged one soak. You can buy cores separately or in bulk, or even salvage them for free from discarded tubes.