I thought the explanation might be found somewhere in the L-Space website, but alas, it's not.
The idea of the clocks made by cuckoos may have (in Sir Terry's) mind and the cuckoos were making a desperate attempt to evolve by being useful.
I wouldn't be surprised if this was right. Just witness what all the bushes and trees and so forth do on the island in
The Last Continent ... or the strange creatures that erupt out of too much belief during
Hogfather. (Or all the Small Gods in, well,
Small Gods, or the spontaneous creation of Anoia).
In Discworld, words and belief have power. If you're in the Mended Drum and you carelessly say, "I believe I'll have another drink", a pint might mysteriously appear in your hand, and the landlord will instantly appear and your elbow and charge you for it. (Given that the Drum isn't far from UU, this may well happen).
