It's for people who need more than 5GB but not 200GB. People in the developing world, or lower income brackets, who can't afford 5-10 USD a month, or people who transfer photos offline instead of keeping their entire camera roll in the cloud.
They open sourced it, so it's just a matter of time now. Linux is still a relatively small amount of their business though so they probably aren't going to make it a priority in-house unfortunately. As a Linux user, I'm well aware that we're still a vocal minority of users
I'm still on google for email and a bit of drive, calendar etc. I've been reading stuff about Proton with some interest as I'd like to ditch google. This doesn't encourage me - what's the point of a mobile only plan? Isn't half the point of a cloud drive to allow sharing with other platforms? I'm just thinking aloud here - I could go read their offerings where I'd probably find that it's their lowest entry level tier and they have less restrictive plans with clients for various platforms?
I would say in that case of making the best use of all their offerings (mail, drive, calendar) Proton unlimited is the way to go: https://proton.me/drive/pricing
I made the switch a year ago from a Google and I'm happy with Unlimited.
But they also have a free tier, good enough to try everything out.
Plus individual subscription options for every one of their products (for example Drive Plus).
Makes no sense a subscription for 20GB / 0,99 a month. You better take a iCloud 50GB subscription for 0,99 a month and use Cryptomator to store your files on iCloud. This is how I do it. Even store Carbon Copy backups (through Cryptomator) this way.