Please explain why is eyebright good for the eyes?

It isn't. According to Web MD, eyeebright: when used directly on the eye, eyebright may not be safe and is not recommended. It can be contaminated and cause eye infections. Side effects of eyebright tincture include confusion, headache, tearing, itching, redness, vision problems, sneezing, nausea, toothache, constipation, cough, trouble breathing, trouble sleeping (insomnia), sweating, and others.