The operation of default-value

[Heime]

I am struggling to understand the operation of "default-value".

The documentation states that

"This is the value that is seen in buffers that do not have their own values
for this variable.  The default value is meaningful for variables with
local bindings in certain buffers."

Take for instance the variable "mode-line-format".  When do buffer variables 
have their own values, and when do they not have them?

How can one figure out what has been set or not (whether buffers have variable
values or not)?.