Being arrested or subject to police violence can be a deeply traumatic, deeply vulnerable experience. As such, it is entirely possible that people subject to these indignities may not want to have that experience broadcast for public consumption. Additionally, if someone is taken into custody, it is preferable that their loved ones not find out by seeing their image plastered across social media.
Aside from the above, there is one other excellent reason to tread carefully when posting footage of someone's arrest or brutalization: opposing counsel. In cases of both arrest and injury, the specter of litigation looms. With an arrest, a person may very well be facing charges. In cases of police repression, the affected party may either be brought up on charges later, or may want to file a lawsuit against the police department.
In any of these scenarios, it is best to offer footage to the person featured, rather than posting it publicly where anyone can see it. In the event of a pending court case, this may involve getting the footage to an incarcerated person's attorney, or perhaps just finding a secure means of giving the data to the person it concerns, thus allowing them to review it (and ideally get legal counsel) before releasing it to the population at large. Even in the absence of a pending court case, reaching out and offering the content to the person(s) depicted, rather than posting it of your own volition, is likely the best course of action.
There are many means by which you can transfer the images you've captured; the best is to put the data onto a thumb drive, and hand it directly to the person you'd like to receive it. Second best is to physically mail a thumb drive, using tamper-evident packaging. In both cases, if for a court case, it is possible that you can get reimbursement from the attorney to whom you are supplying the data. Failing the ability to physically transfer data, you can also resort to email, but should avoid this if possible.
It is very true that it may be difficult to find the person whose arrest or brutalization you've captured, but it is absolutely worth the effort. The photos you snap, and video you record, and your discretion in keeping them private, may be the difference between jail and freedom for someone else.