An important use for DRM in regards to streaming services is to prevent people on shared networks from getting access to the data. For instance, if I was in a college dorm with dorm-wide wifi, and I was watching my de-DRM'd Netflix, anyone on the dorm intranet could snoop my packets and get the data being transmitted to me by Netflix, for free.
That is nigh on impossible to do when the content is encrypted with DRM.