I've seen that too. Which brand are you looking at?
Maybe we can get ahold of their rep and find out what they think it's for.
On some or all one piece shower receptors there is an indentation, on the top of the base, about six inches in from the threshold. Should these be caulked closed at the wall board or left open? This is a discussion on johnbridges.com. Some tile setters feel that this is an air opening to behind the wall and should remain open so that water behind the wall can escape. Other setters and myself feel it should be caulked closed to keep water from getting behind the wallboard and that the indentation is for wall water to be redirected into the drain. What do you think?
Mustee is the brand that started the conversation. I don't read anything in their installation instructions about not caulking. I have sent an email to Sterling and am awaiting a response. At johnbridge the thread is Shower Base Installation in their Professionals Hangout.