[MzxgckZtNqX5i]

Relays can be malicious and try to tamper with the data. Think of Tor relay encryption like Signal's E2E encryption, where the relays are analogous to Signal's servers. You want to ensure they can neither see what you sent (confidentiality) nor modify it without detection (integrity).

[amelius]

Yes, but if it's all encrypted tunnels inside encrypted tunnels (recursively), then those relays can't really see the data, right?

[MzxgckZtNqX5i]

That is correct. But, (in general) encryption does not necessarily guarantees integrity of the data. In other words, a plaintext can be encrypted, the ciphertext given to another party, and they can tamper with the ciphertext in a way that produces predictable changes in the message obtained by decrypting the tampered ciphertext.