Mechanics exist to deter vandals from destroying your stuff. If you have an active claim, only players with the appropriate crime skills can steal/wreck stuff, and doing so will leave crime scents which can be tracked to find the perpetrator. It also applies a temporary debuff to them that prevents them from doing certain things (most notably they can't just teleport home immediately after committing crimes); this further dissuades them from committing crimes unless the result is worth it. Once you have a palisade up and it's dried, people will need to siege you to vandalize/steal your stuff, which is a massive hassle and people will only do it if they really hate you (e.g. you stole/wrecked their stuff) or you have very valuable stuff (which you won't).
As for why it's designed like this,
the developers posted a stickied thread in The Inn of Brodgar detailing the reasons. It's worth a read as it's a very different perspective from what most games go for.