My MIL had a brain haemorrhage aged 58. For the first day or so in hospital she was perfectly lucid then she suddenly went into a coma and we were subsequently told that she was brain dead. Her two children and my FIL were asked about whether they would consent to organ donation as she didn't carry a card. They agreed that anything that could be helpful to others could be used.
We were able to go and sit with her and have our final moments, not knowing if she'd ever know how much she was loved (although we did tell her before she the haemorrhage, of course).
Some time afterwards, my FIL received a letter from the transplant coordinator for the hospital, explaining that both corneas, some skin and her kidneys had been successfully transplanted.
A couple of years later he received another letter, via the coordinator again, from one of the recipients of a kidney.
She'd been on dialysis waiting for a match for ages. The reason she was writing was to say she'd just had a baby, something she and her partner had hoped for but never expected.
That news, whilst not lessening the pain of losing her, did such a lot to show that saying yes to that important question had made such a difference to at least one family. And yes, it did help a lot.