You can get an EZ Pass from different states. Although living in Ohio now, I've kept my New York EZ Pass due to no monthly fees and I have a transponder for my truck and car. I use the same transponder in the truck whether I'm pulling a trailer or not.
Any EZ Pass toll discount depends on the state or municipality in charge of the road. For example, at one time, in NY the toll for the Grand Island Bridges was discounted a little if using EZ Pass. I can't think of any recent trips where there was a discount for using EZ Pass, but since I use it all the time, I really don't pay attention other then to check my monthly statement.
The biggest reason for having EZ Pass is not having to worry about toll money and being able to use "EZ Pass Only" toll lanes and avoiding waiting in line. I would never want to go back to worrying about getting tickets and then digging out toll money when exiting. It's so much easier just having the account linked to a credit card and having it replenished $25 at time.