I got two XD (gonna gift a couple friends this christmas...)
It's not necessary - you can do everything from steam if you want.
It's only useful if you want to bypass steam,
or if you want to continue using your stand-alone version and install the exporters for that version.
I migrated from steam version because I prefer having a clear, backuppable XD installer and install path,
plus I prefer launching Fusion than launching Steam and then Fusion..
but it's really minor hassle
and zero hassle if you already use steam frequently.
This bundle is really crazy!