Well, for money, of course, but perhaps in your own case it might be that fame is the spur. Or you might want to work something out in your head, or you might hope it'll be fun, or you might want to tell the world what a genius you are - or perhaps you want to change the world.
The lawyer, writer, and Member of Parliament Sir Alan Herbert was I'm sure very glad to receive some money for his writing, but he was also out to change the world - and he wanted to have some fun while he was about it. And what gave him (and his readers) a great deal of amusement were the more obscure peculiarities of English Law.
His Misleading Cases, which first appeared as a series in Punch magazine, were explorations of some of the more amazing aspects of the Law, in which Herbert's protagonist Albert Haddock (or, as Haddock insists on appearing in one story about the iniquities of Copyright Law, Haddock, Haddock, Haddock, Haddock, Haddock & Co) appears before Mr Justice Swallow to argue his case.
What laws apply if a car collides with a boat? In law, are snails domestic animals or wild and ferocious beasts?
Among all the fun there are serious points to be made. A P Herbert was genuine in his desire to reform the law (though I shouldn't imagine he was too bothered about snails). He was an important campaigner for new laws on divorce (though his own, apparently very happy, marriage lasted fifty six years) and his Misleading Cases sometimes had pointed things to say about, for instance, defamation, liquor licensing, or the use of the police as agents provocateurs.
Delightfully, AP Herbert's Misleading Cases were several times mistaken for genuine cases by legal experts with no sense of humour; and, even more delightfully, because the stories are firmly based on real law, they have even sometimes been quoted in judicial decisions.
A great hero, A P Herbert, and his stories (and his more serious novels) are terrifically entertaining reads, too.
Word To Use Today: law. This word comes from the Old English lagu, from a Scandinavian word. The Icelandic lög means things laid down.