As the photographer, what guest information can I actually see in that list?

The guest list shows each registered guest's name, email and phone number, along with their avatar initials. That's the contact data you need for follow-ups, print orders, or — with proper consent — WhatsApp notifications when you publish future albums. It's genuinely useful for building your own mailing list of past clients, since every one of these people has already demonstrated they want your photos. Just remember the consent obligations around using their numbers for marketing.