It is more difficult to stand out as unique and different if everyone has a similar background. When I tell people I am a career coach, they find it unique and interesting. When I tell other career coaches I am a career coach, not as exciting.
Get different perspectives. It is more likely that similar professionals will consider issues in a similar way. Get different opinions by speaking to people with varied perspectives.
Competition. Professionals in the same field can feel a natural sense of competition with each other. Break out of that from time to time by meeting different types of people.
Get a broader understanding of how your skills, experience and abilities translate to the overall job market, as opposed to only an understanding from people in your niche.
Being an active member of an association may be very important to you but the more you get out, the more you can bring back to your group.