Why use wireless? Mainly for convenience. You may have a laptop you want to use from any place in your house or patio, with connection to the Internet and to your other computers. You may have several desktops, located upstairs, downstairs, and in the basement, and you want to share files and a single internet connection. Sure, you could have Cat 5 cabling run throughout your house, with jacks on many walls; but this is expensive, and still somewhat inconvient; therefore you use wireless.
The same considerations apply in the office environment. And when you travel, you may want wireless so you can use your laptop at the airport, some hotels, other offices, Starbucks, and various cybecafes.
All the things Melati said are basically true; but he left out one thing. There are ways to provide yourself some protection, especially in the home and office environment. All modern wireless routers that I am familiar with have WEP (wired equivalent privacy) available; this encrypts the transmissions.
Most also make available MAC fitering (that's Media Access Control, not an Apple computer). Every ethernet device, wired or wireless, has a built-in, unique address. In your router you can specify, by these addresses, just which which computers or other devices have access to your system.
You should generaly use both protections. Does this provide you with absolute protection, Absolutely not -- nothing is perfect; but then you have no absolute protection against hackers in any system attached to the Internet. The use of WEP and MAC filtering should, however, reduce your risk to the onder of magnitude of that of a wired internet connection.