What makes this different from a QR code that just links to a website?
It encodes your contact details directly in the vCard format phones already understand, rather than pointing to a page. Scanning it prompts the phone to save a new contact immediately, which works even without an internet connection at the moment of scanning.
What should you leave out?
Only include what you’d hand someone on a printed card. A name, phone number and email cover most cases. Add a company or website only if it’s useful, since more fields make for a denser code that needs to be printed slightly larger to scan reliably.