This and other optical illusions are explained in the video, including the illusion below. Most people aren't capable of seeing all 12 dots in the image at once.
Optical illusions like these are scientifically useful for understanding how eyes and brains function. It highlights shortcomings like the blind spot in the above illusion or ways the brain fills in gaps of information filtered out in an attempt to work more efficiently.
The utility to illusions like these has been tidily shown in lists like the Neural Correlate Society's Optical Illusions of the Year, which comes with explanations about what the illusions reveal.