Buffer sites create one layer of security between your money sites and the PBNs.

You can take it down at any time to remove links pointing back to your money site (manual link disavow)

You don't need it for every project. It is ok to just use PBNs. However, Buffer sites are there to help amplify a general broad content focused PBN into a more relevant content funnel.

Buffer sites can be a free blog site like wordpress.com

Buffer sites only need one post a week to mark it as active in Google’s eyes.