Some street photographers use flash to capture a moment with the contrast of its penetrating light.
I don’t use a flash. This is my personal choice and the only restriction I impose for my work in candid street photography.
There are practical reasons for this restriction. A flash’s light distorts reality and directly impacts the person in the picture. If you worry about confrontation from taking pictures of people, then using flash greatly increases the chance your subject will be upset.
Night scenes require the highest ISO setting that can be used for reasonable image quality. When people are moving it’s impossible to use low shutter speed unless motion blur is your goal.
I try to create a composition that does not depend on details. The mood of the situation, the strong contrast of light emitted by lamps, cars, and advertisements are more important for the photograph.