Colossus isn't for Enigma, it's to attack Lorenz aka "Tunny".

Enigma is among the last of an obsolete YOLO approach to encrypting written text. On the other hand Lorenz, like the systems Britain and her allies were also using, is a stream cipher, not ultimately so different from say ChaCha20 except that Lorenz is broken and ChaCha20 is not. Lorenz uses streams of 5-bit codes whereas today we'd think in 8-bit bytes, but fundamentally it's the same idea.

Attacking this needed a much more sophisticated machine, enter Colossus, a physical realisation of the idea that Computation can be performed by a machine.