Usually I see that in cases where it must be something because in a loop you're concatenating something onto c. And if c is not defined yet you'll get an error, but it it's something, anything, even null, it's okay. Something like
c = [];
for k = 1 : 5
c = [c, k];
end
If we didn't define c as null then the c= line inside the for loop would throw an exception the very first time.
