Those bikes are sometimes called:

• Cargo bikes: straight forward name for their function
• Box bikes: straight forward name for their form

They are good for: moving cargo, transporting kids if they aren't of an age to bike independently, grocery hauls.

They aren't great if you rent on or own a small property as it is tough to secure them with out a fair bit of space.

Other kinds of bikes that are good, especially if you are looking to transport kids, are "long tail" bikes that have extended frames over the rear wheel.

And there are a lot of modifications to regular bikes with front and rear racks that can handle quite a bit, especially with some e-assist too.

None of these options are particularly "cheap". But maybe you want to tell us some of the things you're trying to do with a box bike?