Why do some cows have bells?

What you can't see in this shot is the driving rain, we've had a weather warning for most of the day. It didn't impinge on my world; I've been in meetings or calls almost back-to-back from just past 07:00 - I'm a tad jaded now. I've got some kidneys to cook, so that'll be nice.

And the answer to the question in the subject line? Because the horns don't work. This is the kind of rural information that young me, growing up in an inner-city area, would never have dreamt of knowing.