Share bandwidth and storage of some IPFS clusters, like the Pacman.store
You can learn programming or bisecting and bug reporting and do that. You can package software and maintain packages.
Or did you mean passively use your computer? Seeding shows doesn’t help Linux, you should seed Linux ISOs on Torrent instead. Especially when new versions are released or few people seed.
Bandwidth costs a lot. Manpower is nearly invaluable. Donate your free time and help newbies in chatrooms. Ask maintainers of your favourite software about how you can help. Get involved.
I also use some of my spare time to update OpenStreetMap data in my area.