Not good to drive on!

I'm not sure when or how this damage occurred to my front nearside car tyre (?a big pothole perhaps) but I'm glad it's come to light before causing a problem. It's only just down to the plies but I'd still not be happy to keep using it for long. I went to our local tyre dealer this morning but they've had to order in a replacement of the correct size and type: it'll come tomorrow but I still decided to put the spare wheel on today as I don't want a front wheel blowout.

Why does damage always occur to tyres which have plenty of wear left on the tread?

I reckon it's derelict enough to tag for Marlieske's Derelict Sunday challenge this week.