A couple of examples.
zeros is often used to allocate a variable to a known size, then downstream code will fill in the individual elements. E.g.,
x = zeros(1,5); % Pre-allocate x to the correct size
for k=1:5
x(k) = k^2; % Fill in the individual elements
end
ones could be used to allocate a variable with a constant. E.g.,
x = ones(1,5) * 7; % Allocate a variable and fill it with 7's
