The PC Linkage Device SM-PCE02 is the newer version of the SM-PCE1. Like its older sibling, it can be used to update firmware and customize your Di2 settings. It can also help troubleshoot problems and restore a non-responsive Di2 system. The functionality of this unit is very close to that of the SM-PCE1 - the newer model is faster, a lot smaller and adds a battery check option, allowing you to determine whether any component/unit/bike's power consumption is within reason.
The device has one green LED, one USB port and one E-tube port. The SM-PCE02 and SM-PCE1 cost about the same, which is still a lot more than the Shimano SM-BCR2 battery charger and that also doubles as a PC-interface. The SM-PCE1 and SM-PCE02 are therefore mainly found at bike shops.
What it adds on top of the functionality offered by the regular charger is the ability to connect a single component to it, instead of your entire bike. It also lets you recover crashed components and restore their firmware. Note that the SM-PCE1 should be connected directly to a PC - Shimano recommend you do not use a USB hub.