It’s there, because it’s in the biblatex spec
It allows you to define multiple editors with different roles. E.g. you have one main editor and then you maybe have a secondardy editor whose job is redactor or reviser. It also allows complicated things as Founder (The founding editor of a periodical or a comprehensive publication project such as a
‘Collected Works’ edition or a long-running legal commentary)
See §2.6.3 of the biblatex manual for an explanation of the roles and combinations.
I must admit, it’s the first time I stumbled across this…