You can copy your opponents best creature, perhaps a Titan or a dragon with a powerful enter-the-battlefield effect, swing with it immediately thanks to Haste, and then still have a functional mana rock for your future turns
This is to go with the OG Zendikar ones, or the Expedition frame from Battle for Zendikar
Plus, it's beautiful to look at
It was in large part because the card held some amount of mystique, leading some people to buy in even though they didnt really have a use for it
Sure, a 6/7 vigilance reach dork is a pretty limp threat but if you view it as a free card then bring it on
In other words, the very first card to use typal coupling did so because of flavor