Watercolors are water-based and non-toxic, so paints are thinned with water and paintbrushes can be easily cleaned with soap and water. They are also non-smelly.
Watercolor equipment is all very lightweight, allowing for easy transport. They also will not take up much room in your bag. This is great for plein-air painting! There are even dozens of all-in-one watercolor kits, specifically designed for plein-air painters or artists who travel frequently.