AFAIK you only need an iCloud account to use iCloud related features, like the app store, syncing across devices, access to Apple services, etc. It's very much usable with just a local user.
Admittedly, they do heavily suggesting connecting an iCloud account.
You also literally can't even launch an iPhone past the initial screens without creating/logging into an Apple account. At least Google lets you use an Android phone without forcing an account.