'Know your characters' they said....

Advice to New Writers : "Before starting to write your novel you must know all about your main characters " 

That advice seemed very helpful. 
Back in the day when I was a beginner at novel writing, I diligently wrote a profile for my two main characters, mentioning all the items suggested in the advice.
pet hates, favourite activities, any strengths, flaws, relationship with family, and so on - and on. 
Well, that novel was written long ago but although it earned praise from editors and critiquers,...