In my city, the origin of those laws was partially trying to push the national discourse for immigration reform but mostly due to wanting law enforcement, public health, and education to work better. Without legal paths, you inevitably get people who are joining relatives who are here legally, trying to appeal refugee decisions, etc. where those people are not causing problems but could be victims of crimes everyone wants to be reported and you don’t want perverse incentives like a minor who is a citizen being deprived of education or other support because they have a caretaker who is not a legal resident and doesn’t want to listen to themselves on official forms.