The Sages established Megillah reading to publicize the Purim miracle. The mitzvah is not only private learning; it is public remembrance and proclamation.
In halachic practice, hearing the proper reading can fulfill the obligation of Mikra Megillah. That is why community listening is central on Purim night and day.
Sources: Esther 9:28, Talmud Megillah 4a.