Of course it does something to prevent copying. The idea that DRM doesn't prevent a single copy being made is as absurd as the idea that every single infringing copy made of a work represents a lost sale.
In reality, most infringing copies (other than professional pirates) are casual copies made just because someone can, and most works being copied illegally are recent. If DRM avoids even a modest fraction of those infringing copies for even a relatively short time before it's cracked, it probably still leads to increased legitimate sales as a result and has at least partially succeeded.
Whether it is effective is irrelvant. The question is: Do they have a right to prevent you from copying? If you purchase something, like a video game, I 100% think that no, sellers do not have that right. In netflix's, I think that they do. Whether it's effective shouldn't factor into the equation.. it's an implementation detail.
In reality, most infringing copies (other than professional pirates) are casual copies made just because someone can, and most works being copied illegally are recent. If DRM avoids even a modest fraction of those infringing copies for even a relatively short time before it's cracked, it probably still leads to increased legitimate sales as a result and has at least partially succeeded.