Sometimes it turns up interesting data, like in the last census (2001) 1% of houses in London (yes London) didnt have their own toilet. (ie it was a communal one rather that a private one). Though its usually more common on the Census to ask how many toilets you have rather than do you have one at all (although the US census usually has the do you have a flush toilet on it)
Although the rationale for the question was probably more due to the fact in earlier decades, having your own toilet wasn't as common.
So rather than trying to throw up interesting statistics, they knew beforehand it would be interesting. Finding out now that 99.9% of the population have toilets would be just as interesting, so it will be asked again.