Collaboration will expand your community and your opportunities to connect with people who want and need what you have to offer. Collaboration will also connect you with like-minded people; these connections can go anywhere (from friendships and people who share your stuff to long-term profitable relationships).
Collaboration allows you to be seen in a different light. I mean this in two ways: (1) The person or people you collaborate with will present you in a different way than you would. Whether in an interview, or an introduction, or a promotional tweet, the other person can say things about you that would look ridiculous, conceited, or just plain weird coming from you. (2) You can easily collaborate on something that is a hop outside your normal content—this allows you to pivot into new areas of expertise or simply add to what you already do.