EDIT: Oh, yes, I see, I am not sure why anyone would do this. Maybe someone who had an address or two they want to keep at the lowest possible cost. I'm not sure how email is managed for such accounts, but it's possible Dreamhost will take care of the setup, so maybe you don't have to interact with the web control panel, which might be intimidating for some users. Could also be a legacy arrangement for an older product offering.

Original reply:

Buying domain hosting gives you "unlimited" email addresses, so domain hosting is cheaper as soon as you need 3 or more addresses.

3 addresses x $2 = $6/mo for email vs $4.95/mo for any number of addresses. I have a friend who is married with 4 children. He pays $4.95/mo for 6 addresses. You don't have to actually host an http domain to get email hosting that comes with domain hosting.

"unlimited" in quotes because obviously there are some limits and service terms. You can't start a competitor to gmail with a $4.95/mo hosting account.