Cops are the first step in enforcing the law. Before a judgement is entered, it has to go in front of a judge and maybe a jury depending on the severity. What you said is basically exactly the reason -- you need to have people in the system who can recognize cruelty or injustice in the outcome, gather the whole story, and make balanced judgements. A lot of that isn't really the police's job. It'd be terrible if the police were just going out laying down the law as it was written or however they saw it.
That's not to say there's nothing wrong with the whole system; there's plenty about it that doesn't function well or is bad or needs to be changed (lookin at you plea deals and public defender systems). But I don't at all agree with this simple QED logic with no reference to the reality of how the system exists and how it's evolved to deal with this exact issue starting back many hundreds of years ago when people saw how bad were the problems with having police organizations going around enforcing the law all on their own, and how badly that idea worked out (for this among some other reasons).