Many linux distro isos are available from several different mirrors. Having a secure hash on the original site with the links to mirrors means I don't have to trust the mirror(s).
Another case where the archive hash is useful is when there's some public key crypto involved. I can have a public key from a publisher (gotten either out-of-band or in the past) and the hash can be signed so I can verify it. These schemes would mean that an attacker would at the least need to have compromised a site for an extended period of time (if I have history with the site, the first visit it doesn't do anything extra), or in the case of out-of-band key sharing, multiple communication methods might need to be compromised for an attack to succeed.
But yes, in the common case a hash next to a file link hosted on the same domain really doesn't do anything.
But if someone replaces the archive with a malicious file that decompress normally, he will also probably change the listed checksum on the download page....