Some people do change the default colour in their browser (may not even be deliberate, just playing around with settings). Some browsers may default to grey instead of white. That's why it's important to specify a background colour of white if that's what you want – you can't rely on people having it as a default. Sure, almost all of them will, but there will be some that don't, and for the effort of typing
background-color:white; you might as well make sure that your site will look as (close as possible to what) you intended.
The basic rule is that if you're defining any one of: foreground colour, background colour, link colour, link hover/focus colour, visited link colour, then you should define all of them. Otherwise some people with non-standard defaults will see a complete mess.