Subject: What is the visual 'blind spot'? What causes it? Why do Dr's check it?

I want to understand what causes the "blind spot" in our field 
of vision.  More than that, I want to find out if the size of 
the "blind spot" is the same or different for each person's eyes 
(right and left) and from person to person.  If the size is 
unique to an individual, what makes it unique?  Why do 
ophthamologists check for the size of a person's blind spot?  
What are they looking for?

Re: What is the visual 'blind spot'? What causes it? Why do Dr's check it?