If you have had some hardware replaced, or simply moved your disk into a new PC, and the disk was encrypted with BitLocker, you may find that you are prompted for the recovery key on every boot. This is because on the initial recovery screen it asks for the key simply to access it for the current session (to recover data for example).
If you want to store the key then press Esc on the initial recovery page and enter the recovery key in the subsequent screen – if all goes well the key will be stored and no more prompts will be seen…