I needed to run Migration Assistant to copy files from a Mac Mini with 10.15.7 to another Mac Mini, originally with 10.11.
Time Machine couldn't restore. The APFS drive wouldn't mount. Carbon Copy Cloner couldn't clone the drive either. I started to dd, but realised it wouldn't update the BootROM. I made a Catalina installer with createinstallmedia, to install 10.15.
Migration Assistant still failed to copy over the keychain, because I'd installed 10.15.3 or something on the destination, instead of 10.15.7 (the source Mac Mini). Most files worked, but without the keychain, many passwords and software licence keys weren't available.
I needed the standalone updaters to make both computer run 10.15.7, so I could run Migration Assistant and get the files over (finally!)
It would be a big problem if Apple stop making these available.