Ok, my take on it.
The formation doesnt matter much to me, even if I fancy some. I usually play on Standard mentality, sometimes Positive, but mostly Standard.
Duties allow me to balance the team in more precise way than Mentality does. (Because Mentality change a lot of things).
So let say I'm playing a 4-3-3. If I have: 3xD 5xS 2xA, my team is "balanced" in 3-5-2 with a 4-3-3 formation. With 5 Supports and 3 defend duties, I know the team will play with more possession and solidity than penetration (2 Attack duties), During the highlights, if I see my team having a hard time getting shots, or good shots position, its the game telling me I'm lacking penetration. So I could change 1 Support for 1 Attack duty to add some.
If i'm 1 goal up, and I want to close the game, but without changing Mentality or other thing (tempo, defensive block...), I would change 1 Support for a defensive duty (so balancing the team in a 4-4-2, without changing formation).
I know some roles add penetration or solidity (or both) on their own, but its just an easy way to see if I need to change something, just by looking at what the game is telling me.
Hope it helps.