display property

As I understand it the display property with a value of block will apply
block level formatting to the element it's defined for, e.g. img
{display: block} that being the case why does the recommendation cite
many such examples as:  p{display: block;} is this not redundant since p
is already a block level element and furthermore confusing. First time I
saw this I assumed display:block would apply to the children of the
element containing this property value simply because, why would you
apply display: block to an element that is already a block level
element?