weatherproof housings and conduit and GCFI breakers and all the rest of the things that keep you from getting killed by touching a live wire outside.
Low voltage wire needs to be protected from damage but only so the lighting keeps working, not for safety reasons. That could be nothing more than digging a trench a few inches deep and dropping the cable in.
Low voltage lights can be energy efficient too if you pick the right ones. Halogen lights are bright but short lived. They also burn very hot. The new LED lights last a long time but some poorly made lights can fail pretty quickly due to bad connections or not being waterproof.
If you want to put mood lighting around a good sized deck with 120 VAC lights you could be spending $500, most of that on labor to install all the junction boxes and conduit. You could install low voltage lighting for 1/3 the cost. And it’s safer.
However, if you want to light the back yard to play ball at night low voltage lights won’t work. They can’t produce enough light for that.
So it’s your choice depending on your needs and wants.
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