An offline profile is a record you create and maintain yourself, for somebody who does not have a Vome account. You can assign them opportunities and shifts and record their hours exactly as you would anybody else.
They exist because plenty of the people you work with will never sign up, and your records should not have holes where those people are.
Who maintains the record, and whether the person can be reached in the product.
An offline profile is a real profile in every way that matters for your reporting. It is not a lesser record, it is an unclaimed one.
A first name and a last name are all you strictly need.
No, and you should not invent one.
If the person has no email address, select This profile does not have an email address. The profile is created and works normally.

An email address is only genuinely required if you want to invite that person to claim a Vome account, since the invitation has to go somewhere.
Nearly everything you can do with a Vome User, as long as you are the one doing it. From their profile you can:
Their hours and attendance count in your reports the same as anybody else's.
Anything that depends on somebody being logged in. Worth knowing before you rely on it:
Email is the exception. Where an offline profile does have an email address, shift call-outs and reminders can be sent to them, and you choose whether to email Vome Users, offline profiles, or both.
Yes, on a kiosk. An Attendance Kiosk works for anyone who turns up, with or without a Vome account, which is exactly why it exists. The Attendance QR code is the one that does not work for them, because scanning it requires the mobile app and an account.
You invite them to claim the profile you already built, rather than starting a new one.
You are offered this when you create or import the profile, and you can decline it then and send it later from the Database. Once they claim the account, the record becomes a Vome User and keeps its history, so their past shifts and hours are there waiting for them.
If they registered separately and you have ended up with two records for one person, merge them rather than deleting either.