roast beef?

so i've been living in Boston for a little over 2 years now. in my time here, i've had the pleasure of fully experiencing all of the north shore. during my travels, i've come to notice multiple delicatessens based on one food item - roast beef. it caught my eye the first time i saw a big sign screaming someone's famous roast beef on route 1. what the hell is roast beef anyway?? and why are my fellow new englanders seemingly obsessed with this type of meat? seriously...there's kelly's roast beef, bill and bob's famous roast beef, mike's roast beef, steve's roast beef, jimbo's famous roast beef, banjo's roast beef, billy's famous roast beef, helen's roast beef, liberty bell roast beef, royal roast beef, and the list goes on and on.
i'm not sure the rest of the nation, or even possibly the rest of massachusetts, knows about this cult obsession. or is it that i'm the only oblivious non-roast beefin dude in the area? regardless, this is weird custom, new england.