Target identification is key in any self-defense situation. Ensuring that you are legal to use your firearm can be dependent upon your proper identification of the assailant. Every year there is a negligent shooting of a family member due to improper identification of the target.
Since a significant number of defensive shootings happen in low light and no light situations, part of your daily carry equipment should be a flashlight. Hand-held lights are easy to carry and very affordable. The disadvantage is that now one hand must be occupied holding a flashlight, leaving only one hand for firing. While there are techniques to mitigate this, essentially it is single handed shooting. On the other hand, pistol mounted flashlights are very easy to use and offer the advantage of a two handed grip. Their main disadvantage is added bulk.
Always be safe and remember the 4 gun safety rules!