Invite codes are short and human-pronounceable. They are comprised from a known list of phonetically distinct words. So unlike username/password combinations with specific requirements for length or complexity, they are easier to memorize, communicate and type.
The use of invite codes drastically reduces the chances of making unconscious errors and the subsequent frustration for instance from making typos, forgetting one's password and requiring a reset etc.
Also, invite codes are more secure. As invite codes can only be used once before expiring, their usage helps to protect against certain common attacks for example when credentials are delivered over email, you email provider necessarily gains access to that information (as does anybody else who might access your email account later on). When you use invite codes to receive or exchange credentials, third parties like email provider are excluded from knowing the content - or even the existence - of the information being exchanged.