I use it for a couple of things. One is to get an idea of likely price for an item I have no clue about. Put something up at $100 and if the best offers that you are getting are at $50, that tells you something. Often, I'll start negotiating with the person(s) and see where it goes. When doing this, I use the setting to send me all offers.

Another reason to use it is to select the buyer. Recently listed a Guitar for $900, not a bad price for the axe, but a tad high. Got several offers on it. Chose the guy with 15 years on eBay and 500+ feedback over the guy with 4 months on eBay and 15 feedback.
I used the automatic decline option at $600, as anything under that amount is just a goofball fishing around, not somebody seriously interested in opening a negotiation.

A third thing is for a negotiated package deal on (several) items that the buyer doesn't want to do off eBay. Set the price at $100 for a lot that you've negotiated the price to $50 with the buyer, and then accept that buyer's $50 offer. Prevents you from having an unhappy customer if you just put up the listing and someone else snags it first. (And if someone buys it for twice the negotiated price, I kinda don't care if my original customer is unhappy...)