Correct; because it's a function  which is designed only to return a
value and not carry out any actions. Even if a sub is called from a
function, the action doesn't seem to get carried out.

What are you trying to do?

I thought it would be cool if a custom worksheet function  could
include the equivalent of conditional formatting. That would save a
lot of time compared to setting up conditional formatting as a
separate step. Too bad Excel lets you determine the calling cell's
address, but not really make much use of it.