Regular video bloggers on YouTube can probably attest to situations in which they have uploaded a video to YouTube without realizing they forgot to blur out someone or something that didn't consent to be filmed. It can be very frustrating having to re-edit the original video to blur out the person or logo, and then re-upload it. This is especially true if you didn't catch it very quickly and the video has been up getting hits for some time.
Luckily, Google understood the frustration and made something that rectifies this head-ache. They created a new tool that lets YouTube users blur out any part of a video even after it has already been uploaded. The blur effects can be added, removed, and resized at any time. The tool even has some safeguard measures in place, as viewers will not be able to turn off the effect. Well that's just handy! Why didn't they think of that sooner?
Source: YouTube Blog