In case no one is home when burglars try to get into your home, these lights will also play an important role by lighting up the area where it detected movement and then people from the outside can now see clearly what is going on in your home and they now can call the police.
However, like other devices these security lights also have a downside. One is that they are actually not foolproof, but it is a lot better to get and install one rather than have no protection at all.