But what has a pocket-watch got to do with God?

The pocket-watch is a metaphor. Anselm is really talking about the whole world. Anselm is saying that, like the pocket-watch, the world is a finely tuned instrument. The whole world runs like clockwork. Seasons come and go, like clockwork; animals migrate at the same time, and to the same place, every year, like clockwork. But Anselm's point is that it is surprising that the whole world runs in a predictable and ordered way, like that. This fact requires explanation. The world is being compared to the pocket-watch, in that they both behave in a predictable and ordered way. But, there is an important difference between them. In the case of the pocket-watch we have an explanation of why the pocket-watch behaves in a predictable and ordered way. The pocket-watch was designed to behave in that way, by its creator - the clockmaker. Anselm uses the metaphor with the pocket-watch to suggest an explanation of the predictable and ordered nature of the world: like the pocket-watch, the world has an intelligent creator - God.