While for the most part police are great, we see in the news that from time to time some of them abuse their power. This has getting more exposure lately because of people recording these incidents with their cell phones and then uploading them to Youtube. It's also useful for filming crimes which may be happening, after a traffic accident to get evidence of the damage, or even capturing baby's first steps. We almost always have our cellphones on our person, or close at hand, and having a camera on them is great for this reason.
For other features, I'll let this post from Failbook explain: