Not a stupid question. It is used to compete with micro algae for nutrients. This in turn leads to less algae in the display tank. The one thing to remember with macro algae is to trim it down. The nutrients are locked up by the macro algae as it grows, but if it dies then it gets released back into the water. So when you trim it, you are removing the bad stuff.
The other use for macro algae is as food for fish like tangs. Of course this depends on the type of macro algae being grown. It can also add color to the tank (like a planted freshwater tank).
The final use I can think of is as a place for pods to multiply. Maybe others will come up with more reasons to have macro algae, but these are mine.