its good for both stamina and weight loss actually mate.
when your going at a constant level your muscles and your anabolism/metabolism level out at a constant level, and you start to retain energy. when you change the intensity like you do with HIIt, your constantly giving your muscles the "trick", causing more calorie loss (good if your trying to lose weight) and a better way to build muscle (ask any bodybuilder, tricking muscle fibres is the way to new growth)