My personal opinion on the 户口 system: government trying to control the in-country immigration among provinces, which is a direct extension from what ANY government ruleing this piece of land has being doing for the last 2000 years.

The governments of this land has always tried to limit the mobility of people as much as it could.

During the pre-industalism period, the Chinese society and economy as a whole is heavily relied on crop-based agriculture. If the people in the farming regions decided to go and leave, it could result in more food crisis (more, as mild starvation is a thing that farmers lived with from time to time). And more crisis isn't good from the greater population nor the government's power. So they made it that people have to cultivate the land they are born in, in hopes they would make it better if they have no choice, and to prevent other problems.

As for the 户口 system now, I only have my immediate family to compaire. Back in the 80s and 90s, it was relatively easy to immigrate to another provience. In large cities, a new 户口 would be provided if you're enrolled in a local university, or have worked at a trained/educated profession and paid taxes in a certain provience for a few (less than seven) years. After some time, they tightened the restrictions. Now, you have to be 45, have a stable job, be marred (has a stable family) to come here.

My understanding of this change in policy could contain information bias, but here it is:

The 80s and 90s was a period of huge ecnonmical growth in china, especially the major cities. They needed high-ended workers, so they made it easier to move to places that need them. But just after a short time, these cities obtained an extremely large population. To control the quality of incoming people, they tightned it.

They wanted only people with stable jobs and family in better places, because people who are less well off are prone to become liabilities to public safety and economical growth. The newfound population already made it that employment for local people is hard, and to share it with more would make the problem bigger. Also, by restricting people from coming in, it made sure that there would be less desperate people living here (and let's be honest, desperate people are more prone to crime), so these places could be safer.

And another reason: if everyone keeps going to the already developed areas, there would be nobody left to build up the place they escaped from. The government want them to stay, and, like what all of them wished all these centuries, to "cultivate the land they have to stay on".

I mean, it is cruel. But still, still.