The thing that impresses people the most is... Ditto has 70 HP. Obviously that isn't enough to avoid a OHKO from major attackers, but early game that's amazing if the Basic you want to play is something like Tynamo. You drop your Ditto(s) and next turn Transform into Tynamo(s) and then you can immediately Evolve the Tynamo(s), so that you never got stuck with a tiny 40 HP Fighting Weak Basic Pokemon in play.
Are there other uses? Certainly. Many decks will run a single clutch attacker because it has an important Ability, Attack, and/or Type. Especially if your deck is focused on this, Ditto becomes a placeholder. I can "prep" Ditto as I would that key Pokemon, even if I am already using that key Pokemon. Opponent Fighting Weak but dishing out OHKOs? One Terrakion can become up to five... provided your deck has room for the Dittos.
It can take some finesse, though; you need to retrieve whatever was discarded so Ditto can Transform into it. Still, it beats having to wait yet another turn, for instance, to get the needed energy onto a recycled Terrakion.
Lastly, it allows you to surprise an opponent while obviously building something, including adapting to changing strategies. Not such a big deal when you're running something that can already move Energy around easily, but handy nonetheless. As an example, a Ditto you were prepping to become a Terrakion can just as easily be Transformed into a Mewtwo EX instead.