it applies equally to age, disability, race and so on
Also, assuming employers are collecting for some purpose, not just because everybody else does, it should help an employer know that within the area they are based how sucessful they have been in recruiting to reflect the community they are based.
For instance if say they are located in an inner city area where a number of different communities live, then it should show how sucessful their outreach has been to that community.
That is why employers, with out saying it is perfect, should use the categories that match the census ones, so that you can then compare your recruitment response to the locality.
But I suspect many just go through this as a paper exercise and dont do anything with it at all. And if that is the case they shouldn't collect in the first place.