Yep agree with the two above and it also protects your service users, or customers.
In practical terms in could mean if you work in Social Services, or any other personal service, you could be leaving around information that someone has HIV, or mental health issues, or has suffered some form of abuse etc, for all to see.
In the commercial world it might be someone's bank account details, or security information, handy info such as you are always out on a Wednesday evening because, for example, you always play Champion's League Football....oh, you already knew that....