this has probably been asked plenty of times.... but is say xanathars and volo's guide included in the subscription service on dnd beyond? if not..... what Is the point?

Subscription doesn’t include any books. It gives you more website features but all the books are independent of any sub.

The point as defined by the subscription description:

removes ads, allows you to create an unlimited number of characters, and add publicly-shared homebrew content to your collection to use

Many people (including myself) don't feel the need to purchase a subscription for these features.

A master tier subscription gives you all the above benefits plus the ability to enable content sharing on campaigns to share any purchased books within the group with the group. Every play group that has purchased books on DDB should have 1 person with this sub.

Hero tier is just to remove ads (which a lot of people do normally), allow an unlimited number of characters to be saved (for when you really like theorycrafting) and allow homebrew content others have been made to be saved to your collection.

Master tier is all of that, plus it allows you to enable content sharing in campaigns