There are probably two reasons for building a layout in module format.
The first is for your own personal use. We've all been in the situation where we've spent a lot of time building a nice layout in the basement. And then we decide to move and we have to junk everything that we've built. If you build your layout in modular sections, it's relatively easy to dismantle the layout and move.
The second reason for building a layout in modular format is to have a hell of a lot of fun running trains with a bunch of guys on a huge layout which you could never even dream of building in your lifetime. Each person builds a module or a series of modules according to specific standards - height, width, length, track location, wiring, control systems, operating rules, waybills, dispatchers, etc, etc.
Using Ottawa Valley HOTRAK
http://www.hotrak.ca as an example (simply because I belong to HOTRAK), we get together 6 - 8 times a year. We rent a big hall. We put out a call for modules. Based upon the response, one of our members "designs" the layout. This is simply a matter of dragging and dropping the available modules into some layout planning software (XTrCad) and making sure that things fit.
On the Friday evening of our meet, everyone brings their modules to the hall. The modules are assembled according to the layout plan. Because the modules are built to standards, the tracks between modules connect together, the track power connects together (4-wire trailer plugs), and the control system (Digitrax DCC) connects together. A couple of "section foremen" make sure that things are being pulled together and that assembly proceeds at a reasonable pace.
By Saturday morning, we are ready to "play trains". We have a short debriefing session, trains assignments are handed out (or rather they are requested), we strap on the headsets for communicating with the dispatcher, and we get the trains rolling. Trains usually run until 9:30 pm on Saturday. We start again Sunday morning around 8am. We pull the plug around 4pm. By 5:30 pm you'd never know that we had occupied that hall the whole weekend. By 6:30pm (I have a bit of a distance to get home), my car is unloaded, my modules packed away, and I'm off with my wife to our favourite restaurant for supper. When I get home, I'm like the kid who tried to eat the whole thing. I don't want to look at the trains for a couple of weeks. I'm overstuffed with running trains. And I've got the biggest grin on my face - much like the cat that ate the canary.