Thanks for asking the question!
The XT tiers are if you need or want to play a live mix (or playlist) for longer than just 1 hour, i.e. in the case of the XT *h tier, *up to 8 hours*.
If you have an XT tier then you can still stop a mix playing at any time you want, but you have the flexibility to play it for longer.
If you are playing a live mix for a long time, then of course you would want to have your device plugged into a power source.
An example use case for when you might want longer play times is to provide a live music background during the day for an art installation or a small shop; you might want to use an iPod touch or iPad for that. Of course, you might also want to use it to make a really long continuous recording.
Hoping that helps explain what they are for!
NB: There is one other totally unintended "use case" and it is for "super fans". If you are a "super fan" and really love what we are doing and wished there was a way to help support our efforts by somehow giving a little extra (we are in a small niche remember), then this is it: you can view the tiers as a kind of "tip jar". Even if you did not need the extra Music Play Time, you could get one of the XT tiers.
. We especially thank any super fan who decides to do this, but we of course love *all* our customers.