Every time you create a key, you get a different one. So don't delete it except when you are intentionally replacing it, and even then proceed with caution - perhaps as described below.

If your stuff is encrypted and you lose the key, you can't decrypt it; if you are changing your key and/or your certificate make sure you have the new key, update by decrypting with the old key and encrypting with the new key (and then checking it's all ok) before dropping the old one. And again, add a column so you don't do it in place, then rename the columns so that the old one has a different name and the new one has the name the old one used to have, and check that everything still works; if it does, then you can delete the (renamed) old column and the old key and certificate.