Here are some good reasons to join.
From a readers perspective, I feel Quora is the best place to get answers for your all sort of doubts and questions. Whether it's regarding your education or your personal life or professional life or business advice, you will get the answer to everything. Just put your question or Quora and request the right person. You can even be anonymous in case you don’t want to reveal your identity.
From a writers point of view, I feel Quora is the best place to share your knowledge and experience. Even if you teach in a school you can’t share your knowledge with as many people as on Quora. You can make a video and share on FB or YouTube but again making videos is not everyone’s cup of tea. You can share your experience via writing Books or Blogs but again it's not easy to reach the large audience as a beginner. But on quora even if you are writing your first answer Quora’s algorithm will help you to get the maximum audience.