They make great watch dogs and protectors of their family, home, and even livestock due to their size and deep bark. Our first Leo was huge, 205 pounds, and protected us quite well. He also stopped protecting when told to stand down. That is what you want. And you want a Leo who has met all kinds of people throughout their first year of life. Do they make good guard dogs? No. And they should not be encouraged to do so as the Leo is considered a friendly and gentle breed.
Leonbergers are much more protective of the females and children in a family than the male in charge. The male in charge is considered by a Leonberger to be the Alpha of his pack thus should not need his protection as much. Their huge size along with their deep bark is usually enough to deter almost anyone with some common sense.