

First reported by TechCrunch back in March, the port is located underneath one of the fasteners used for watchbands. Although it hasn't been confirmed what the port is used for, most assume that Apple professionals would use it for running diagnostics tests on the Watch. This could easily be the reason why the port is still included in retail versions.
The MacRumors tipster was told by the Apple employee at his try-on appointment that he or she received no official training for the port. Previous reports claimed that Apple might be saving the port, for either first-party or third-party accessories, like "smart" bands or other things that could interface with the Apple Watch.