Every actor or actress, during the course of their career will find it beneficial to join an actors' union, for several reasons, up to and including the benefits such as medical coverage and access to a credit union, however, many beginning actors and actresses also join the union before they are actually ready to do so and find later that it was a mistake to jump in so quickly without knowing the pros and cons of joining.

There are three major Unions that cover diverse areas of the acting profession, those being:

Actors' Equity Association (AEA) who represent theater actors and stage managers.

The American Federation of Television and Radio Artists,(AFTRA) who represent those who are part of the news and broadcasting media, entertainment programming, records and recording, and commercials and industrial as well as educational media.

The Screen Actors' Guild (SAG) who represents the actors and actresses in films.

Joining any one of the unions can provide great benefits such as health care and access to a credit union, but there are some drawbacks too, as there are with anything.

One of the major things a young actor can do that will prevent his or her development, will be to join a union before they are ready to do so.

Once they join a union, it does tend to lend them some credibility, but it also provides that they may work only for a union company or at a union job.

While this is positive in some ways, in others it is less so.
It becomes a catch 22 situation.
You can't work for anyone but a union company yet you don't have enough experience that a union company wants to hire you for their project.
It tends to mean that they are auditioning against those who have most likely been at the job for a longer time, who are far more qualified and have more experience.
It is really a step in the right direction to build a solid resume prior to joining a union so that when you have done so, you have a solid chance of getting the jobs with the union companies because you have enough experience in the field.

The point is, that before you join the union, think carefully. If you have projects in the works that are not union jobs, then take some time prior to joining.
It would be a great boon and a good asset, yes, to hold the union card.
It tends to make you feel better about what you're doing, but it can be less than beneficial to you if you're not really ready, resume wise.
Make some careful consideration before you make the decision about accepting an offered union card.

The point of all this is to think carefully before you join a union. It feels great to carry that card and feel that you've made it, but it can also be a burden. Assess where you are in your career carefully before you make your decision.