You can also say this if you want to stop and not continue any further. Like when a conversation becomes too uncomfortable "sorry fellows, this is where I get off"

It can be a question and this has two meanings. One: The place where you stop/ get off a form of transport as this is where you want to be. Two: It might be when someone is trying to pick a fight with you. "Where do you get off?" Is a demanding confrontation where you are asking what the person's problem with you is. Hope that helps.