Cops exist for two reasons: (1) to bringing any and every potential criminal in front of a court and (2) protect the property of those with the most influence (elites tied to major commerce/government).

• Amusing themselves by harassing people.

• Ensuring those of lower social status 'know their place' through loophole abuse and outright illegal activity.

• Ensuring no cop ever faces justice if it can be helped, unless it's something really bad. Like being a whistleblower, or trying to stop sexual harassment in the department.

So many uses!

Policing can be a useful function of government when they fulfill (1), but oftentimes, they don't even do that much. At least here in the US, a great number of police departments are so crooked and unsupervised that they may as well be legal gangs.

Oh good, they can come in 20-40 minutes and take a report after the fact, if they feel up to it. Maybe they can shame the victim for being raped while they're at it.

Look, there are American police departments out there with good internal cultures. Ones that take community policing and social justice seriously. The problem isn't that cops are inherently bad people, usually. ACAB is applicable only in two senses - either one is a literal anarchist who believes that exercise of state authority in any form makes cops inherently bastards - or one is somewhat dramatically condemning the (overwhelming) tendency of cops to 'close ranks' and protect their own, or ignore the abuses of their fellows.

The problem is that institutions which lack oversight or responsibilities inevitably devolve into self-serving and self-perpetuating entities. Due to the hagiography of police in the past half-century, practical oversight has been sorely lacking, and many departments have devolved, are run by the devolved, and have the employing standards in the hands of the devolved. Not to mention the usual problems that come with any concentration of power. Furthermore, the legal requirements of police to respect the rights of citizens and to uphold the law has been continuously whittled away by SCOTUS decisions.

For many people, an encounter with a cop is terrifying - because it's a crapshoot whether you're going to end up with a violent thug, a simple jerk, or a professional. Poor standards, poor enforcement (ironic), poor cultures, poor assignation of legal responsibilities, poor selection of candidates, all of these things contribute. And because of that, in many areas people don't call the police - because they don't trust the police to do anything meaningful. And in many areas, they may be right.