It's obvious that every job role should have a clearly defined description of its purpose, responsibilities and tasks - otherwise how can anyone be clear on what you require of them or how well they are doing.
The problem comes when layer upon layer of responsibilities get added in - sometimes to make the job look more interesting or fulfilling other times to dump al the stuff that no one else wants to do!.
I came across a job description recently with 20 responsibilities - assuming about 200 working days and 50% productivity that means 5 days a year on each - how does the employee know where to focus? Answer, they don't so they have to make up their own mind and tackle what they enjoy most, feel like or guess to be important - they can't do it all anyway. The net result is that whilst the employee may excel at some of the elements they are always a failure at others - hardly motivating.
So always ask the question when drafting a job description - what does this person need to fulfil to merit a pay rise, bonus or promotion and what if they failed to achieve would I fire them for? What is the single most important deliverable that makes this role worthwhile for both sides. With that clarity both of you will know what is essential and can work on it constructively.