www.Digikey.com
they are not exactly soldered onto the carrier, they are formed from a single piece of material, so one of the benefits is they tend to be very close to each other in value (where impedance matching is important)
other than that, its just a convenient package when you need 4, 6, 7 or 8 resistors all with the same value, like pullup resistors on a bus for example.