They happen when a mutual organisation (like a Building Society) turns itself into a commercial one (like a bank). The owners (ie account holders) got cash and ceased to be owners.
The 'bank' then goes and demonstrates it doesn't know how to be a bank and gets rescued by a larger organisation and its shareholders lose out
In the current market situation there are unlikely to be any - in future, who knows. The only people who would get them would be those who had accounts before the 'donate to charity' rule became virtually universal. And the charities.
It's nothing to be concerned about