How to use the immigration policy in Age of Empires 3?

The effect of the immigration policy is that after the completion of R&D, you can add a population of 100 without building a house, but the total population of 200 people will not increase, which means you will save 1000 trees. But this technology is basically useless, because it was only developed in the imperial era (AD 5). It's already late in the game. It is estimated that your population has reached 200, so it is basically useless.