1. Insurance doesn't always cover ADHD medication, and when it does it can be a pain; every time my employer switched insurers I had to change my medication because the new insurer wouldn't cover the same thing the previous insurer covered. I've never had insurance cover my psychiatrist either, which amortizes out to about $100/mo.
2. Stimulant medication improves symptoms for over 80% of people with ADHD but "normalizes" about 1/3. So If you are in the 2/3 people that still have some ADHD symptoms on medication, then you're still going to have to cultivate healthy coping mechanisms.
3. Sometimes you go on a trip and forget to bring your medication, or you forget to bring the monthly prescription into the pharmacy on time (no refills on Schedule II substances), or your pharm is out of your meds. Now you are temporarily unmedicated and still need to function.