The benefits are many. As indicated, professionalism is key. Opportunities to have access to seminars and conferences in terms of information and sponsorship, civic responsibilities, job opportunities and career building, are just a few.
I found the experience at the Microbiology Society UK extremely beneficial and rewarding. I was able to take part in committee meetings, my first conference, and meet people willing to support my future endeavours. It is a great place to develop a network and learn what it means to be a 'professional' scientist.