There are many different reasons. One is that the union is classified as a self-help organization. The members are making the decisions and running their chapter. Another reason to join is to show leadership. Employers may notice employees who may have a leadership role in the union and promote them or hire them in as a supervisor because they have shown the skill to lead.
Social pressure is another reason that individuals join the union.
There are no US laws that require employees to join a union, however in most states the employer can agree with a union. If after a certain period of time, the employee has not joined the union they are fired. Joining a union generally includes paying dues, a ceremony, and a membership pledge. There are also many reasons that employees decide not to join a union. Some have anti-union attitudes and believe the union is unnecessary.