Why Keep Cats

Have you ever thought about why people bother to keep cats as pets?

I did just the other morning. It had been a rather damp night of the kind that brings out of hiding the many slugs and snails that inhabit my garden. I live with four cats and it is their habit to have me feed them first thing in the morning when I get up. Breakfast had been served and consumed and the furry ones had taken themselves outside, presumably to engage in their favourite activity of nature study.
With my first cup of coffee of the day in hand, I followed them. Standing on the step of the back door I looked out on my neglected and pest-ridden garden and three of the cats who were sitting, a few feet apart, in a line alone the path. Creeping around them on the still damp flagstones were dozens of snails.

Those cats, I thought, spend much of their day terrorising the local wild life. Killing birds, mice, voles, shrews, moles, rats, rabbits, anything they can get hold of really. But the pests that do real damage i.e. slugs and snails (especially snails) they ignore. So why do I keep them? What are they for?