At night you may need a flash because you do not have enough available light to make a good exposure of your subject.
During the day you may need a flash to add light to the darker areas of your subject.
Night or day, your flash is a valuable tool that can add light whenever and wherever you need it.
If you think your flash is too bright, you can make it darker.
If you think your flash is too dark you can make it brighter.
Learning to use flash is important because it allows you to add an additional, controllable source of light to your picture whenever you need it.