Emotional stability. My girlfriend had massive ups and downs, almost bipolar; the medication (slow release, concerta or its cheaper but equivalent alternative) evened things out. I have a friend who doesn't want the diagnosis or treatment because he's fine with it, and you can tell he struggles with keeping his emotions under control. He mainly gets really frustrated and loses his patience if things aren't going as fast as he wants to.
The other one is getting overwhelmed and/or having meltdowns, because one comorbidity of ADHD is sensory processing disorder. It's things like grocery stores with music and children screaming that can get too much.
Another one is things being "loud / busy" in your head, maddeningly so. I've heard that taking the meds quiets things down, and it's a relief.
Anyway, a diagnosis won't hurt, it'll improve your self-awareness. You won't be forced to take medication or anything if you don't want to. A diagnosis is usually a questionnaire and a "take this ritalin and tell me how you feel in a bit".