The signature is generated using public key cryptography [1] meaning, to generate a valid signature, you have to have the private key.

Because the signature only has to be generated when a new version of the file is released, you can use the most bothersome (and effective) manual methods of security, like keeping the private key on an air gapped [2] computer.

So if an attacker manages to switch the downloaded file for a malicious one, they may still not have got access to the private key used for release signing.

Of course, the attacker could still remove all links to the signature file and all mention of it in the public documentation, so downloaders wouldn't know there was a signature to check. So it's better if the site isn't compromised in the first place.

[1] https://en.wikipedia.org/wiki/Public-key_cryptography [2] https://en.wikipedia.org/wiki/Air_gap_%28networking%29