Also a beta is usually the first release that customers can install.
In the early stages of developing a software product (alpha) you obviously would never give the code to customers as it is too unreliable.
Once the product has become fairly reliable you release the "beta" code to customers to allow them to test it or use it.
As the product gets more reliable you release another beta, then another, so you have Beta1, Beta2, Beta3 etc, each one getting better and better (you hope).
Eventually the product gets finished and you build the "gold" code and release it for sale to the public.