So when we were first watching I got super excited when we noticed that there were 9 episodes this season instead of 8 like in season 1...and then episode 7 happened.
I already said this, but to go into it more...I think they had to have this episode, just to go more into Eleven's past (and I think they will/should unpack more of her past as the show goes forward). I think we needed to know more about her mom and what happened to her, and I personally have been curious to know if there's a 001-010 out there (maybe even beyond 011?). I do think the whole "will she leave Hawkins to go on a killing rampage with the Suicide Squad?" part of it was a little dumb, because obviously she wasn't going to do that. And I thought that the characters in the new gang were AWFUL. Eight didn't bother me that much, and neither did the driver/enforcer characters, but the other two looked and acted just SO ridiculous. Especially Mohawk Guy, OMG was he annoying. I totally think the Eleven backstory was necessary but I think they could have done it a different way.
But as others have said, I think they did it to show 1.) how she can really use anger to be more powerful, 2.) that she did have this opportunity to get back at the Hawkins Lab people and decided not to go that route, 3.) that Brenner may still be alive (??! - we saw the Demogorgon jump on him in season 1 but never saw his body afterward), and 4.) that of the 3 "homes" she had offered to her (Aunt Becky's house, Chicago or Hawkins), she chose Hawkins. At this point I'm guessing that Brenner really is alive and that part of how the show will end (hopefully after 5 seasons!!) will be an Eleven vs. Brenner showdown.
I did like the part where she made the decision to go back to Hawkins after remembering Mike "naming" her and Hopper having breakfast with her. That was super sweet and I'm looking forward to more Eleven/Mike and Eleven/Hopper moments (and really some Mike/Hopper moments too - I think they have some issues to sort through with Hopper keeping Eleven a secret all year and Mike and Eleven likely dating in S3 - the whole "what are your intentions with my daughter?"-type conversation). And I may have already said this but I DO NOT like Eleven's punk makeover! I thought she was so cute with the overalls and curly hair.