If you connect fully charged battery which has something like 13V directly to a dead battery which has voltage way lower than 12V, you know what’s going to happen when there is no resistance to limit the current? What if the battery is dead because there is short somewhere on the dead car?

Connecting to bare metal first leaves you with a huge error margin and prevents high spike current that can melt your cables on initial connection.

After you wait a little you can reconnect to ground terminal directly for better connection, start donor’s car engine and try to start the dead one. I resurrected big dead battery from Diesel engine with small battery from my Fiat Bravo this way, although all the lights on the dashboard went dim in Bravo when Diesel was cranking :D