Door knocking isn't for everyone. It's a traditional form of marketing that takes a lot of time and requires a one-to-one conversation. But if it's done right, it can be every bit as effective as more popular modern marketing techniques. Lead generation, especially early in your career, will likely be your biggest challenge.
Door knocking is a way to develop leads beyond your sphere of influence. It's true that email, website or billboard advertising allow you to reach a larger audience with less effort, but there's really no replacing the value of looking someone in the eye, introducing yourself and shaking hands. Even in our high-tech society, people still appreciate a personal conversation and they're much more likely to do business with someone they've met and liked in person than someone that sent them an email.