How are Hash Trees useful?

I was reading on Wikipedia about hash trees, and I don't understand the benefits or purposes of this structure - they seem to require more hashes than just one per leaf with no significant use of the extra hashes.

For example, the use case on wikipedia is that they are used to validate data received in a P2P system. But why is this better than having a one-to-one mapping of blocks numbers and their hashes, without having the tree structure?

Could someone please explain how and why hash trees are useful?

Thanks in advance,

Moshe