HarryB Is there at least one single reason to do it anyway?
You might want to use Tor or a VPN for system updates if you want to hide the fact that you are using GrapheneOS to your ISP. This might improve your anonymity against an adversary that tries to correlate anonymous online activity with a specific device or internet user, as your anonymity set is larger. This might also help you avoid unwanted attention in case you are in a country where GrapheneOS legality might be questionable. The only reason to use Tor over a VPN here is because Tor is free of charge.
You might also want to use Tor or a VPN to make sure your device cannot be targetted with a custom tailored GrapheneOS update image, in case GrapheneOS release signing key has been compromised, or the GrapheneOS team have been legally coerced to sign a compromised image and deliver it to a specific user. If they don't know your IP address or any other unique device identifier, they cannot know who to deliver the compromised update to, and would have to deliver it to everyone, greatly increasing the risk that the compromise will be detected by the community. Tor generally provides more protection in this case than a VPN, as the Tor exit node changes and is randomly chosen by the software, and not by you.
vincente213 Tor exit nodes are already stretched as it is without you downloading GOS updates over it.
Actually, the GrapheneOS updates are usually very small, since they are delta coded images against reproducibly built full images. So it won't be that much of a burden. The Tor network also has quite a lot of capacity nowadays, and can handle quite large loads of traffic. It was way worse in the past. QubesOS offer as an option to download all OS and template updates over Tor, so this is also already done by other OSes.
But it is always good to pay for the services one use, for example by donating to the Tor project or Tor relay operators.