This is a great middle game Policy to use if you have big cities whose Housing limit has been reached. Before the Neighborhood there are few opportunities to increase Housing beyond tile improvements, and this Policy is one of them (although 1 Housing isn't such a big deal, it's still something). You can use the Policy in one of your Economic slots on a more-or-less permanent basis, and swap it with something else as the strategic situation dictates. The Population growth in the city will be boosted while the Policy is active, and stunted while it's not.
Note that recently founded cities won't benefit from Insulae - you need at least 2 Districts in the city in order for it to apply. So, having care with starting Fresh Water supplies for a city is still paramount.
Once people came pouring in on all those roads that led to Rome, they had to live someplace in the urban sprawl; thus, the 'insulae,' a kind of apartment building built of stone and concrete (which the Romans invented), evolved as housing for the lower and middle class plebs. Five or six stories high (although some were larger), the ground floor was taken up by shops with residential apartments upstairs … the best being those nearest to the ground. But the insulae were prone to collapse, and fire was an ever-present danger. Augustus and later Nero set construction standards; nevertheless, by the 4th Century it is estimated that there were around 42,000 to 46,000 insulae in the city.