This is where you can add Partner Keys that somebody else has given you. Depending on the permissions your partner assigned to the key, these keys may enable you to access your partner's account directly through the 'Switch Account' functionality, or otherwise collaborate with your partner through system tickets.
You don't have to add a key in order to use a guest access link. If you don't plan on using it often, you can continue to access your partner's account by clicking the emailed link each time you want to do so.
The person who gave you the Partner Key may revoke your access at any time. Expired or revoked keys are not automatically removed from this list, but will indicate they are no longer valid and will not appear under the 'Switch Account' functionality.
You may assign a friendly name to each Partner Key you add, but this is only to make it easier for you to tell them apart. The key value is given to you by somebody else. Only the key value is important for you to have access to their account.