First of all, it's not an "E-brake Light." It's a Brake System Warning Light.
The light illuminates when your PARKING brake is set, to WARN you that your parking brake is set. It will also illuminate if there is a malfunction, or excessive wear in your braking system. The brake fluid level directly corresponds with brake pad wear. If your brake pads are worn out, and your brake fluid reads low, then the light will illuminate to tell you to have the brakes checked out ASAP.
It's like a Check Engine light for your brake system. It serves MANY purposes.