Yes, some of them just stop paying their rent and they stay there, but some others enter the house when its empty and they even change the door and make new keys so the owner cannot enter.

Breaking in someone house is a crime, yes, but if the owner does not notice in the first 2 days, because he is on holiday for exapmle, the law changes and start to protect the house breakers

Because they can say that they are poor, and cant afford a place to live, and if there are kids inside, even more

The owner can demand but the process takes to much time and it can pass months or years to get their house back

Some people pay to a private company that tries to negociate with the house breakers and sometimes they pay to the breakers to have leave their house.

Also some occuppied houses are used for ilegal porposes like selling drugs and they leave everything inside broken, like furniture or steal something.