Still in my first year of photography so still figuring things out but I do a lot of long exposure and street/buildings. I recently saw/read something about using a single flash to light up buildings for architecture/real estate photography and figured it was about time I bought a flash. But I also remember reading flashes are generally shit for low light photography.

What are the main uses for a flash?
Are there things that you can't do well without a flash?
Is there always a better alternative to using a flash?
Any general rules of thumb for when to use and when not to use flash?
What are some lesser known cool uses of flash? Isolation/flash drop-off for macro or portrait photography using a narrow aperture and no background, for example

General flash input appreciated.