I could understand epic since I think that's mainly for doctors but don't most hospital systems use MyChart or similar portals nowadays to let patients access access their appointments/payment/lab results/doctor notes/etc
Epic is a electronic health record (EHR) software used by many hospitals and clinics. It's not the only EHR there are many but it's probably the most known. Doctors usually type patient notes, diagnoses, prescriptions... everything goes in Epic and your team of providers can gain access to see that info.
Not really. MyChart (which is provided by Epic) is a way for doctors and clinics to communicate with patients. Although you could give you doctor access to your information on Kate's App, that's not the purpose, and they probably don't want it.
As someone who never heard of either MyChart nor Epic, I'm guessing it could be useful for people like me who don't have those things.