> What does reading a great novel or starting a garden specifically accomplish?

It accomplishes many things - specifically granting entertainment, pleasure, etc that practitioners like.

> It seems to me that you are starting from the viewpoint that everything has to prove its worth before you accept it

I'm starting with the viewpoint that there are literally thousands of various different practices out there have have existed for a long time and have been practiced by many people. Many of these are complete bullshit. How do you filter out the good from the bad/useless?

> even if millions of people before you have found it fulfilling and worthwhile

Millions of people have found many many different things fulfilling and worthwhile over the ages, some of these things we've since realized are bullshit/bad. Do you accept every single belief/practice based on how popular it has been?

> If you had never read a book before, and someone was trying to convince you to try it, what could they point to that would fulfill all your criteria?

They could say: it's entertaining/interesting/pleasurable, they could say that knowledge/insights are contained in books, that different/interesting perspectives and other people's thoughts are contained in books (which are objective facts), etc. Saying 'it makes you smarter' is vague and unconvincing.