Being part of the union means that you will have access to a lot of information for free. The union was created to protect the rights of musicians. In cases where you are just starting out in the music industry it will cost a lot of money to draw up music contracts and get legal advice specific to the music industry. There are also people available to advise the musicians on issues such as performance contracts for the session artists. These kinds of contracts can be hard to get from the regular lawyers or not be specific enough to give you the kind of advice needed in the industry. If you join the musician union you will have full access to such information.
Another benefit of being members of the union is the protection you get under the public liability insurance. This insurance covers the musician up to $10 million. The insurance is meant to cover musician in the event that public member is harmed in the process. Many promoters or venues will ask for the evidence of insurance it therefore helps if you already have the insurance in place from the union. It is a cost effective way to have the insurance without paying the high costs all by yourself.