I did mine a couple of weeks ago.
It's a much about checking who is liable to pay council tax. Councils are having a big crackdown on people who claim single occupancy reductions.
I was caught out by this a few years ago, when I didn't return my form. My daughter no longer lived with me and I should have informed the council. The next thing I knew, I was being threatened with a court order for back payment of council tax. I was receiving child tax credits at the time and I was also informed that I needed to be reassessed, because there was another adult living in my house, even though there wasn't.
Roses is right. Councils have to keep an up-to-date list by law.