The reason I went looking for the diagnosis was 10+ years of utter misery. I had been on various treatments for depression & anxiety, but found them minimally effective. I was getting by but was pretty apathetic about life. Over the years I'd investigated various other mental illnesses, but never came across one that seemed to fit me. I came across ADHD purely by chance, when an acquaintance was diagnosed and I looked it up. Previously I had probably ignored it assuming that it was just for hyperactive kids.
This one did seem to fit, and frankly I was desperate. The though of a pill that could actually fix some of my issues made it worth a try at the very least.
For the past two years I've been happier than any other time in my life that I can remember. Life was just easier. The most telling point for me was when I realised that I was no longer apathetic about life, I actively wanted to live, I had things that I wanted to do, goals to achieve - and I was actually able to do them.
I don't know if I actually have ADHD, I still question the diagnosis. It worries me sometimes that maybe I don't have it and that instead I'm just not very good at life. The truth is that it doesn't really matter, the diagnosis and treatment worked - If I payed for a placebo I don't really care.
Perhaps your family member is the same - maybe they have ADHD, maybe they just needed something to help them through life, maybe they had run out of alternatives - it just gets to a point where you wonder 'how much does it matter even if I'm wrong?'