I was almost at the point of retiring the app since Mail & Calendar has supported iCalendar (.ics) files for a while now. Then I came across a scenario where the app is still required. vCalendar files (.vcs) are very similar but use version 1.0 of the vCalendar spec whereas iCalendar is version 2.0. Mail & Calendar still can’t open this format at the time of writing.
I’ve updated the UWP app for Creators Update and later and it now just registers for .vcs files (all .ics files will be handled by the Calendar app and this avoid the OS popping up a choose app dialog unnecessarily). Doing the update also made me check that it could work on Surface Hub and HoloLens (although it just uses the standard 2D Calendar UI). If you get .vcs files via email attachments or online this is a really simple (and free) app to import them into the Windows Calendar.
Download from the store for all Windows 10 devices:-