Why Get a Death Certificate

Some people wonder why should they get a death certificate for a ancestor especially if they got the quarter & year they died, well here is what I have found by getting them.
First my grandparent on the 1911 census it showed that they had 3 children 2 had died & one was still alive (my mum born in May of that year) so I did a search & found twins & had got the birth certificates & a search also gave me their deaths, which sadly both died after a few hours, the informant was a Alice Dartnell, who it turns out to be my grandmother's sister Alice so now I know her married name & could find her marriage the sad part on all of this was that that within a month or so of my Grandmother's 2 boys dying Alice gave birth & her son also died.
On another certificate which is for Doris Gertrude Barnes nee Gillings she was the 2nd wife of Jessie Frederick Barnes.
She died in 1978 & it gives the informant as son Peter & maiden name as Fisk well first her mother's maiden name was Fiske so why that mistake has happen I did get the GRO to check & it was correct the other thing was Jessie & Doris didn't have any children, but from someone who in contact with the Doris's side of the family has said  that she adopted 3 children, sadly i don't know much more than that as but the death certificate shows that a son was the informant.
I have on other death certificates found out about family members, I do find that this happens more when the spouse of the person has already passed as normally the spouse is the one who registers the death.