In previous years, I was fortunate enough to teach in a portable. A trailer. Or, if you like, a "modular classroom." For some, this might not sound like a great deal. Isn't a "real" classroom---one inside the building---much better?
There are some advantages to being inside the school building, but not many. Out in the portable, you're mistress (or master) of your own destiny. You control the climate. You can turn off the speaker if you don't want to be bothered with announcements. All that hallway noise and other distractions from outside your classroom? Not a problem.
Modular classrooms are not supposed to be permanent solutions, although that is usually what happens. The one I taught in is nearly 25 years old. Since it was not designed to be a permanent structure, there are issues with the stability of the floor and a rather frightening collection of mold growing in the walls. But if you're a quick growing school or district, you don't have a lot of choice in terms of using portable classrooms. Permanent structures require lots of capital. Trailers are ~$100,000 each. (I wrote about funding problems for school buildings a few months ago.)
I don't know about laws in every state, but in Washington, when a new school building is proposed, it has to be built around current enrollment---not future projections. It can be a few years in between the start of such a project and the first day of school there. What you end up with (as in a case in our district) is a brand new building...and 10 portables sitting outside because the school isn't large enough.
The Boston Globe published an article this week on these "Educational Building Blocks." (id: firstname.lastname@example.org; password: bugmenot) Apart from the teachers who actually use these spaces, those quoted in the article are unhappy with having to use modular classrooms for more than a few years. None of the naysayers seem to have a (financial) plan for getting rid of them.
Some schools do what they can in order to help these mobile structures blend in with the rest of the campus. Others, like one elementary in my district, embrace the "trailer" aspect and put plastic flamingos and other ephemera outside. Whatever the attitude, these temporary structures aren't going away from your area school anytime soon.