I apologize if this has been brought up. I haven't read the whole thread. I don't understand the point of announcing that you're a sanctuary city, and besides, it has no legal meaning that I know of.
Where I live there are quite a few illegal immigrants. Law enforcement wants them to know they can come forward if they have evidence of a serious crime. A murderer or rapist for example. Perhaps the murderer preys on illegal immigrants, believing they're more likely to get away with crimes. I used to cover city government - it's been a long time - but I think it's still policy that cops simply don't ask people about their immigration status if they are reporting crime. And the immigrants know this, so they can share their information without fear. Local cops have enough on their plates without having to be immigration cops.
There is also special visa for witnesses in criminal cases.