Ari wrote Music Theory for the Bass Player because she noticed something striking in many bassists: many intermediate self-taught bassists teach themselves through learning songs via TAB tend to reach a very typical plateau and then they get insufficient help to break through to the next level. 
Here is where they are often at:

• they find themselves feeling like a copycat
• they long to be more creative and free all across the fretboard, to be able to add interesting fills, and create their own grooves! 
• Most importantly, they want the thrill of agency and self expression!

(All this is true for beginners, too, btw!)

Typically there comes a moment when they realize that theory might be a key to deeper knowledge and understanding.