I just got my Bronica ETRS kit, and was browsing KEH and Ffordes to see which accessories I could put on my wish list. I noticed that you can get both 120E and 120EI backs. What is the difference between these?

Also, you can get backs for 35mm film? I can understand why someone wants to toy around with the 135W, as the format is wider than normal 35mm, but why would you use the 135N? Why not just bring your 35mm slr?