The letter also says that if you want to complete it early, then you need to notify them if anything changes.
I presume it spreads the load on the website if some, who don't expect their circumstances to change, fill it in early.
As for those in hospital, or other places, they are asked to ensure that they appear on the census at their usual address. As a student nurse in 1971, I contacted a few next-of-kin to check, and I remember a discussion with the homeless hostel next door about some patients.
In 1981, I was on a long-distance coach. The driver handed us all slips of paper asking us to ensure that we were on a census form at our usual address.