It hasn't been confirmed but it's highly likely the X has a liquidmetal back. Apple's had an exclusive arrangement for quite awhile and this would be the first significant use of it.
Primarily yes but, to be clear, liquidmetal is a (metallic) glass.
So yes Apple's main reason is for wireless charging but they also want to differentiate where they can and liquidmetal gives them "the hardest glass in a smartphone ever." (Ive quote).
Analysis and background including a very relevent patent application made a year ago and published in March for "using Liquid Metal (Metallic Glass) for the Backside of an iPhone":
