The bag is inert
Paper or mesh, and it does nothing to the flavour. What is inside it is a commercial decision, and the decision is nearly always small-particle material because that is what extracts in three minutes. The format sets the requirement and the requirement sets the cut.
Why small particles extract faster
More surface area in contact with water. That is also why they extract bitterness faster, which is why a bag left in too long goes harsh sooner than loose leaf does. Neither is a defect; both follow from the same physics.
Where it costs you
Two cases. Flowers, because milling destroys the structure you would use to judge them — whole chamomile heads are assessable and chamomile dust is not. And aromatic leaves, whose character sits in surface oil that oxidises faster once the leaf is broken.
Where it does not
Roots, barks and roasted grains are cut or roasted whatever you buy. Nobody assesses chicory by eye. Buying these loose gains you dosing control and very little else, and the honest advice is that the argument is weaker here.
The middle option
An empty fill-your-own filter bag: your material, chosen and assessed, in a format that behaves like a bag. It costs almost nothing and is the answer for anyone who wants whole flowers and a three-minute cup.