How umbrellas work is that they are either for shoot through, or for reflecting the light. The latter can be either a silver reflector, that will cast a strong and hard light, or another reflecting umbrella type, which can be for example a golden colour. This will give your subject a nicer look in skin tones.
The shoot through are mainly made of a white translucent material, it will soften the light, and because they are so easy to take with you, they are good to have in a travel kit, for shooting portraits outside. Maybe using a portable studio setup, consisting of speed lights, with some selected light modifiers, where the umbrellas are good because they are lite and very useful. It is a wedding photographer chosen tool.