I have just received a bundle of copies of family history material found when my late aunt died (in 2013). Including copies of some certificates which I already have... except.
Why might somebody apply for a English copy birth certificate in the first half of the 20th century?
My grandfather Robert Wright (b 1896 d 1967). Somebody (unknown) applied to Somerset House (where the UK GRO used to be) and a copy birth certificate was issued on 1 June 1926. There's no obvious event in his descendants' history that would prompt a requirement for his birth certificate, and his mother didn't die until 1932. However his father's death is unknown (sometime after between 1924 and maybe 1932) so this may be a factor.
My grandmother Dorothy Brookes (b 1896, d 1980). Somebody (equally unknown) applied to Somerset House and a copy birth certificate was issued on 27 June 1959. Again, why? there's no obvious precipitating family event (her siblings all died before she married; and her children were all married and the dob of all her grandchildren do not coincide).