I'd like to offer an in-universe answer.
Owen Lars, Luke's uncle, is a very conservative person. Living on a relatively outlaw world with species aggressive towards humans such as the Sand People, he is focussed on his farm.
He's been given responsibility for Luke, almost certainly knowing his true parentage, he tries to bring him up as a farmer and tries to discourage his tendency to want to leave. At the start of the movie there is an exchange about him going to the Academy, which Owen refuses:
OWEN You must understand I need you here, Luke.
LUKE But it's a whole 'nother year.
OWEN Look, it's only one more season.
LUKE Yeah, that's what you said last year when Biggs and Tank left.
Some of this fear of the unknown and conservative nature has rubbed off on Luke. When Ben/Obi-wan suggests that Luke accompany him taking the plans to Alderaan, he is suddenly nervous:
LUKE I can't get involved! I've got work to do! It's not that I like
the Empire. I hate it! But there's nothing I can do about it right
now. It's such a long way from here.
BEN That's your uncle talking.
LUKE Oh boy, my uncle. How am I ever going to explain this?
BEN Learn about the Force, Luke.
LUKE Look, I can take you as far as Anchorhead. You can get a
transport there to Mos Eisley or wherever you're going.
Anchorhead is the nearest small town to the farm, but given that Mos Eisley is a spaceport and Owen is nervous about Luke leaving it seems very likely that Luke has never been allowed to travel as far as Mos Eisley.
So when Ben and Luke stop and we see them overlooking the city, it's very likely that this is the first time Luke has been there. Ben isn't explaining that Mos Eisley is a space port, or where it is. It's clear that Luke already knew that it was the nearest place to get off-planet.
Everything we've seen about Luke so far is that he has lived a very sheltered life, a product of his upbringing on a remote farm by an arguably overprotective uncle. Ben's line is just a comment, giving Luke a warning that this place is rougher than he is used to.