How my times have you taken a bag of cockles out for a session to then re-freeze most? Get them out for another trip and then repeat the process, only to find they have become the consistency of snot and fall of the hook?
Sure you could use them for burley or even chuck 'em in the bin. However with a bit of time and a bag of salt you can salt your own to further use as bait.
Why would you? They are much tougher holding on the hook more effectively, don't slime your fingers up and will last several re-freezes well. Plus you save a bit of money (cockles aren't cheap these days) and less