Why tea re-steeps and a flower does not
A rolled tea leaf unfurls over several infusions and releases in stages. A dried flower gives most of what it has in the first three minutes, and a second steep is usually thin water with the bitterness left in it.
What does re-steep
Roots and barks, because a first simmer does not exhaust dense woody material — ginseng, liquorice and cassia will all give a usable second pass. Rolled kuding leaves are sold on the assumption of repeated refills through a day. Yerba mate is refilled continuously by design, which is the whole shape of the practice.
What does not
Flowers, aromatic leaves, ground spice and dried fruit. If you want more, use more material rather than more water — the second steep of chamomile is not weak chamomile, it is a different and worse drink.
The grain case
Roasted grains sit in between. A second pass is genuinely drinkable and noticeably lighter, and in Japanese and Korean households a jug is often topped up once before the material is discarded.