What reason would there be to give up British citizenship?
The only one I can think of, is to aquire a different citizenship that doesn't allow dual citizens (like China). Although even then, the guidance on the British government site suggests they will go along with whatever crap China wants (like destroying a British passport) but allow keeping British citizenship, issuing a new passport afterwards, if requested to.
This was a very big deal during earlier days of Brexit discussions, as dual German-British citizens were in a pickle - German constitution only allows German citizens to hold a citizenship of another EU country, which the UK was going to stop being. So German citizens were faced with a choice - they had to revoke one of their citizenships. Fortunately the German government passed emergency legislation which allowed the British citizens to retain their German citizenship as well.
And then, well, someone times people renounce because they don't live there and they don't agree with the policies of that country.
There was an immigration case a few months ago which involved a person born in Northern Ireland attempting to use an immigration scheme for her husband which was only available to EU Nationals.
Because the British State conferred British Citizenship onto this person upon merely being born in Northern Ireland, despite this person identifying solely as Irish, they were unable to avail of this scheme.
The UK Home Office told this person to renounce their citizenship if they wished to available of this scheme.
(This was all basically nullified by the EU Settlement Scheme coming up to Brexit, but it'll probably rear it's head again)
TL;DR Dual National wanted to avail of an EU Scheme, which was unable to be used by a British Citizen. One solution was to renounce British Citizenship.
The only one I can think of, is to aquire a different citizenship that doesn't allow dual citizens (like China). Although even then, the guidance on the British government site suggests they will go along with whatever crap China wants (like destroying a British passport) but allow keeping British citizenship, issuing a new passport afterwards, if requested to.