Why claim a terrain on singleplayer mode?
Is there any advantage of paying taxes to own land if you are the only non-NPC player?
This https://steamcommunity.com/app/333950/discussions/0/2592234299547126328/ has a reply from the developers stating that is primarily for multiplayer only.
it's mostly for multiplayer gameplay, if you pay the taxes (at the claim block) it protects all creations (house/castle) in your area.
Aside from this, reading over the https://medievalengineerswiki.com/w/Claim_Block doesn't seem to indicate anything useful for singleplayer.