What's the Point?

So today I went to lunch with a friend.  We went to Cafe Rio.  A semi-casual dining establishment.  The kinds where you stand in a line, get to the counter and order your food.  Drinks are from a cardboard cup filled at the drink station.
My friend and I both just wanted water.  The drink station was swarming with people, due to the fact that it was lunch hour for the business park near by.  The water button is the kind you push down with your finger right in front of the lemonade.  We both filled up and ate lunch.   On the way out we both wanted a refill.  Once again it was swamped by people.  I filled up my cup and noticed my friend filling hers up too.  I looked closer at the button she was pushing.  It was marked soda.  I asked my friend if she knew she was getting plain soda water, she hadn't noticed.

Then it hit me,  I have NEVER seen anyone fill up with just plain soda water.  Seriously, what's the point?  Can you imagine "Yes dear, I'd love a refill, just plain soda water".  I don't get it.