The Most important Reason for Sanctuary cities is Public Safety

If people who are living somewhere don't report crimes from murders, to violence, to gang rapes to abuse to whatever then EVERYONE is not safe.

In a Sanctuary city there is an implicit Agreement that illegals (non-citizens) will not be targeted for arrest as non-citizens and in return they will feel safe to report crimes to the local authorities. This then makes these cities like:

Los Angeles
Chicago
San Francisco

And many many more across the U.S.

Safe for both citizens and non-citizens.

Without Sanctuary cities it would be just outright Gang warfare, gang Rapes, mass casualties etc.

Sanctuary cities do not exist primarily for the reasons people think. They are primarily for the public safety of both citizens and non-citizens.