I think it’s a useful sanity check. Let’s say you have an engineering department of, say, 5 teams. All of the leads are straight white cis men. That’s could very well be because it’s a small company yadda yadda. But it’s good to keep track of it as the company grows to 10, 15, 20 teams. If your engineering department large and you have literally no minorities or what minorities you hire quickly quit, that should be a red flag that something is going wrong with your company culture. Otherwise you’ll end up having a hit piece about how an engineering director sexually harassed women and HR covered for it (NYT about Uber, Google has had this scandal, blizzard is being investigated right now, riot games had a piece published, etc.)

Edit: that’s not to say all companies with large engineering departments filled with one kind of gender/race/sexuality are always bad. But it should be examined to ensure it isn’t a result of a legally vulnerable workplace.

For "demographic workforce data" it's legally required: https://www.eeoc.gov/employers/eeo-1-data-collection

Also in order to establish the level of diversity in the company, they need to count how many of each kind of person there is.