Dogs are just a proxy for the racist, hateful attitudes of the police. After all, their accuracy is no better than a coin toss [1]. It's clear they are there so the police can claim they're not racist/hateful/etc. while still being so. Any other claim is instantly disproved by both science and common sense.
1. https://www.dailykos.com/story/2015/8/7/1409751/-Court-rules...