There's is a very simple answer to that: Notebooks are getting stolen more and more frequently. Close to a million notebooks were stolen in the USA last year according to recent study of PWC, also over 750.000 PC's were stolen. We can ignore this by saying, this is not America, but the fact remains. Notebook hardware is easily replaced, and as a good professional user you will have a recent backup available to restore the information stored on your notebook, but what risk are you willing to take that the information on your hard drive fall in the wrong hands?

But I have a password activated in Windows…
Nice, but Windows passwords do not really protect unauthorised access; they merely make it a bit difficult to gain access. A lot of tools are available to reset Windows passwords regardless the version of Windows you are using. Another method is to put your hard drive in another computer next to an existing hard drive and all the information on your hard drive is accessible.

But I have a BIOS password activated in my notebook…
Nice too, but BIOS passwords are also not hard to erase, most notebooks have a ‘secret’ BIOS reset button in the notebook, the only thing you have to do is to unscrew a couple of screws and there you are…