As Geriaviator says, it was built to work and as earlier stated modified from another Moth to fulfil requirements.
As such the two things which made it great are the ease of repair and the handling characteristics.
Of the handling, well, it's handling is poor. And that makes it a great trainer!
But still today, decades later, it is such a rewarding aircraft to fly well and great fun. And it is still flying!
She is such a seductive mistress. We know her and her foibles and failings. But we keep coming back and we love her anyway.