Try something like this
body { font-family: Verdana }
table { font-family: Arial }
against
* { font-family: Verdana }
table { font-family: Arial }
And see which styles are applied to the cells of the table.
There is a difference between "applied to the whole document" and "applied to every element of the document" when you're dealing with cascading stylesheets.
Applying a cascading style to the body applies it to all tags within body until a tag overwrites it. Then the overwritten style is applied to all tags within that one.
However, there are some styles that don't cascade, such as margin and padding (usually where it makes no sense to). Those can only be applied to specific tags and that's where a wildcard can come in useful (albeit rarely).
Most non-cascading styles also have a value of inherit (eg. margin: inherit), which means "take the parent tag's values".