The main reason is that he Association offers a platform to represent the interests of mathematical physicists worldwide through which you can exchange ideas, discuss concerns and inform the broader scientific community and the general public about our achievements.
The IAMP is as strong as their members make it. We invite you to join because we think that you share our common interests in promoting mathematical physics. Your becoming a member would be beneficial for you and for our community as well.
Looking at the membership from the practical point of view, several things are useful to mention:
It is easy to become an Association member, check the "Joining IAMP" link at www.iamp.org.