The word "This"

[mrthlinh]

There’s this guy who sits in front of me on the bus sometimes.

I don't know why they use the word "this" here. Why don't they just say "There’s a guy who sits in front of me on the bus sometimes.

Thanks for any replies.

[CarloSsS]

NOT A TEACHER:
I would say that the meaning is the same with both "this" and "a", although this usage of "this" is informal. And I'm pretty sure that it's AmE only.

[Raymott]

NOT A TEACHER:
I would say that the meaning is the same with both "this" and "a", although this usage of "this" is informal. And I'm pretty sure that it's AmE only.

No, it's not AmE only.

To mrthlinh
You can think of it being idiomatic.