Using party passcodes allows guests to view their RSVP without typing in their name. While there are certainly a number of benefits of choosing the option of to have guests use a party passcode and proceed directly to Step 2 (e.g. more streamlined experience, mitigates risk guest misspell their name and see error message, automatically pipes in your guest(s) names in step 2, greater security, etc), it's important to understand the consequences.
When using this feature, guests will not see the Welcome Text (via Setup > Form Design) which may be an issue if you're displaying event itinerary and/or other important details.
Setup > Form Design
Here is an example of an event that included a lot of important details in the Welcome Text guests see in Step 1. Had this event host have used Unique Passcodes they would not have seen these details (everything below the logo in header and above the guest information fields):