If someone gets a scan of your passport they could use it for identity theft. People want a scan of your passport to verify your identity. A scammer could use it for the same purpose if they have other useful information like your address, and government IDs such as social security number.
A copy of your passport has useful data for an attacker, like your birth date and place of birth. Place of birth is often offered as an option for security questions for banking or password recovery. And of course there's your picture as well. So if an identity thief got a copy of your passport they would have a some useful data on you right away.
That being said it's sometimes unavoidable to give out a copy of your passport or other identity documents. It may be a requirement for a job, for example. The best you can do is offer more secure ways to send the data (faxing, snail mail, hand delivery, etc), and be vigilant for identity theft. Don't use anything available on your passport as security questions either.