Why use the complicated formula to rotate a point instead of just $x=x\cos(deg)$ and $y=y\sin(deg)$?

Question

I apologize in advance for the silly question, but I'm wondering why one would promote the following formula for rotating a point

x = (x * cos(deg)) + (y * sin(deg));
y = (x * sin(deg)) + (y * cos(deg));

when we can use this one, which is obviously simpler:

x = x * cos(deg);
y = y * sin(deg);

What are the reasons why most people would prefer the first formula over the second one?