I would break down outdoor lighting into two parts.
1. Attached to the house lighting:
This is sometimes for security, sometimes aesthetics, sometimes both. More toward security, often motion sensor lights are used. These lights are almost always done with 120 volts.
2. Not attached to the house lighting:
This could be in a garden, yard, walkway, around a driveway, around a deck, etc. These lights are simpler and saver to do with 12 volts.