Model Languages are a fun thing to study. Model languaging is similar to model railroading, only, instead of building model railroads and showing them off in your basement, you build model languages and show them off in your writing. And it's cheaper too.
JRR Tolkien was said to have discovered his love of languages at a young age while travelling by train and watching the elaborate names of the Welsh train stations passing by1. He realised that language was a living, changing thing and, in his Middle Earth books, invented around 20 languages solely for his characters.
Most writers haven't gone to such extremes, sometimes only inventing a basic scheme for names or places.
Language on the Internet
Conlang, artificial language, constructed language, fictional language, model language, whatever – language builders can't really agree on the name to call their pastime because for many years 'closet linguists' were not able to communicate2. With the Internet, a new era in linguistics is beginning.
The Internet has brought more people interested in linguistics and grammar to the model languaging front. LangMaker.Com, for example, has become a hub/portal for model languaging enthusiasts, and no matter which term you use, you can find news and information, and even get hold of LangMaker, software especially devised for quickly inventing and keeping track of your own language.
Rick Morneau's essays on artificial language design are great forays into topics of interest to those designing a language.
Mark Rosenfelder provides a Language Construction Kit for the artificial language builder, and gives a brief overview of all those things that a language builder will need to know. Richard Kennaway's constructed languages list is a great annotated list of language links.
What's it For?
There are practical reasons for studying grammar, or creating model languages. First, grammar gives us an idea of how the mind works. You see, grammar really isn't just a system of rules, but a fundamental piece of the brain's design that we have been studying for centuries in order to better understand ourselves.
As for practical model languages; there are many that were built for entirely pragmatic reasons. Esperanto for instance, an experiment at creating an international language, Loglan/Lojban3, an experiment at defining a truly logical and unambiguous language, and others are out there. Each one is an experiment, testing the bounds of language or the human mind, or a multi-national experiment to break the culture barriers.
Another recent practical model language is UNL, a project co-founded by the United Nations University to allow easier translation to/from native languages... The project looks as if it will be using linguistics in an interesting fashion, but if it works it has to be seen.