A bit pointless then if your not covered for property worked on. What happens when you crack some glass with your hot water? Why do you buy a policy that doesn’t cover you for property worked on?
As Daz says, it's really if you injure someone, somehow. Drop a pole on their head or something; but a local window cleaner to me did sub a job out to a family member and they had a set of ladders blow from the side of a wall and land on someone's head. He posted about it here a few years back. I'm not sure how that turned out, but that would be the kind of situation you'd use this insurance for.
Otherwise it's a tick-box exercise for some commercial jobs that require their window cleaners to be insured. And you can also use it as a selling point.
Wor Lass has recently been DBS checked (for something non-window cleaning related), but I think I'll get DBS checked too and use that also.
The perception that window cleaners are drug-taking scallywags can be useful if you appear not to be (which most aren't, I know).