How is the earthworm useful to farmers?

Earthworms enrich the soil with natural fertilizer.

‘Well,’ the Earthworm said. ‘Next time you stand in a field or in a garden and look around you, then just remember this: that every grain of soil upon the surface of the land, every tiny little bit of soil that you can see has actually passed through the body of an Earthworm during the last few years! Isn’t that wonderful?’
‘It’s not possible!’ said James.
‘My dear boy, it’s a fact.’
‘You mean you actually swallow soil?’
‘Like mad,’ the Earthworm said proudly. ‘In one end and out the other.’
‘But what’s the point?’
‘What do you mean, what’s the point?’
‘Why do you do it?’
‘We do it for the farmers. It makes the soil nice and light and crumbly so that things will grow well in it. If you really want to know, the farmers couldn’t do without us. We are essential. We are vital. So it is only natural that the farmer should love us. He loves us even more, I believe, than he loves the Ladybird.’