If you provide your own custom generated ICS file hosted at a URL it's nice to be able to give Google Calendar users an easy way to subscribe to that feed.
As far as I can tell this isn't documented anywhere, but it is possible.
The format is https://www.google.com/calendar/render?cid=webcal:// followed by your URL.
For example: https://www.google.com/calendar/render?cid=webcal://pretalx.com/pycon-au-2020/schedule/export/schedule.ics
This drops the user into the a Google Calendar interface with a prompt for them to confirm their subscription.
I believe this only works if you are serving your ICS feed over HTTPS.
Unfortunately as far as I can tell Google Calendar polls for updates to the feed less than once every 24 hours. This is frustrating, as it makes this feature far less useful than it could be.