reefer - because, as imperfect as it is - and I do know the results will not give any exact figures for all kinds of things - it is the best estimate we we will get for for the total population and whatever other breakdowns can be extracted from it. And the numbers are a sight better than the regular use of more general, and occasionally inflammatory, phrases relating to immigration which are based on one's own impressions and press and other media reports. the numbers are not 100% accurate, but they do allow you to put things in proportion.
An example might be better ...
There was a youtube video called Muslim Demographics a couple of years ago (it's probably still there) which claimed that the Muslim population of the UK had risen 30-fold in 30 years. they got that figure by estimating that there were around 82,000 Muslims here some 30 years ago and that there are now about 2.5 million.
BUT there was no question about religious belief on the census till 2001. However if you take the 1981 census and use the number of people born in Pakistan and Bangladesh as the baseline, the number of Muslims in the UK was around 300,000. Using that as a start point and the estimate of 2.5 million now, you get a bit more than an 8-fold increase not a 30-fold one.
The numbers may not be totally accurate, but they are a lot better than nothing to counteract some of the codswallop that gets tossed around as fact.