Good sir OP, are you aware of the marksmen's damage and survival conditions? I shall remind you for the sake of discussion.
They must not have someone standing ontop of them and they must not take too much damage, preferably they must also not be CC'ed.
Now i ask, what exactly is Janna as specialist at doing? Shields to prevent damage and boost the marksmans damage, heals, to keep the healbar up in the green, a knockback to prevent engages, and a knockup to disrupt any and nearly all attacks.
She has everything marksmen need of a support to succeed, the peeling, the healing, the shielding and the CC'ing.
Even a garbage marksman that knows what to build and how to move around can succeed with a good janna protecting them.
Assassins are negated, divers are nearly all negated (except those with unstoppable abilties) mages are severely reduced in terms of effectiveness and tanks are prevented from gapclosing on the marksman.
All this done by Janna, she reduces the threat lvl of a enemy team towards her marksman extremely well just by default.
Why? Because she covers all the weaknesses of the class, lack of mobility, lack of durability, lack of self peel and lack of dive prevention.
It's not that Janna will always be good as long as marksmen are, rather, marksmen will always be good as long as Janna is.
This is what we call carrying.