Question

What's the point of Ditto having the Imposter ability?

Answer 1

Imposter allows Ditto to automatically transform into the Pokémon that it is facing. It benefits Ditto, as Ditto no longer needs to waste a turn using transform to turn it's self into the intended Pokémon.

Source: Experience

Hope I helped :)

Comments

Ok, that makes sense.

Answer 2

It can reconstitute its entire cellular structure to change into what it sees, but it returns to normal when it relaxes.

Ditto is essentially a being compromised of cells which can fuse themselves to copy various objects, people, or animals. Imposter is an extension of it's description, it changes on sight because it has no true form of its own.