Duplicate Member Databases?

Hello,
Applying a corvid-based signup lightbox, I’ve Been trying to understand, looking at quite every example I could, Is there a reasonable answer why corvid developers tend to manage double member databases?

  1. Site dashboard - An inherent WIX site-member management.

  2. A member content collection - Actively corvid-based duplicated Member database.

Reference examples:

Isn’t the dashboard database enough? What am I missing?
Thanks