What does this relay do?

Mine is missing on my 01 xk8

 

It is missing on most XKs. If provides ignition switched power to the 3-prong black connector next to the battery and also behind the glovebox if it is installed.

 

Howdy Folks:
Per WhiteXKR's Heads up warning to me about a year ago when I rewired my Sirius receiver, It is worth saying again.... Make sure you use a Jaguar relay or one with diode protection across the relay coil to protect the cars electronics from transient back voltage if you decide to add this relay.