I’ve never seen a clearer statement on the subject:

Here’s what’s driving the whole patent engine – public companies are held legally responsible to maximize profit for their shareholders. To the extent that if they fail to generate profit (or acceptably sufficient profit) they can be sued by shareholders. The board of directors is liable. Which drives public companies to squeeze every possible gram of cash out of any possible revenue stream. If they failed to get the patent for a product, and someone else managed to get it – and then came after the company – shareholders would be pissed. So, software patents are conceptually an arms race. Companies are filing patent applications for anything they can think of in order to both protect themselves from others doing the same thing, and to maximize profit by handing the patent to their lawyers and owning a market segment as a result.
http://www.darcynorman.net/2006/08/02/software-patents-and-legally-required-greed