Just as carrying an extra magazine can be beneficial for your everyday carry loadout, carrying a handheld or weapon mounted flashlight can be of invaluable benefit. Whether it is handheld or weapon mounted, there are numerous instances in which you could find it useful.
Perfect example - your home or place of work loses power during the middle of the day and you have found yourself unable to see, or it is late when you leave work and you hear a suspicious noise by your vehicle. In both of these instances, a small handheld flashlight could easily flood the area with light to help you determine the best next steps to take in the situation.
A situation where a weapon mounted light is beneficial is if someone enters your home in the middle of the night you will be able to flood the person with light to identify if they are a threat or if they are a friend or family member trying to slip in without waking you. These weapon mounted lights may also be very useful if you are going into a lit area from a dark area. The light will enable you to see well as you enter the darker area.
These are just a few instances in which having either type or both types of lights may be beneficial to you. Think about your normal day and consider adding one or both to your loadout. Remember, it is always better to have it and not need it rather than need it and not have it!