These devices provide direct attached storage that can be used to save user content, such as in a backup, to move content from place to place faster than a network allows or even for expanding the useful storage capacity for some common household devices such as some set top boxes or DVRs.
While many people do use their garage primarily forstorage, and park their cars on the street, more expect to have aplace to keep their expensive wheeled investments out of the elements.
And you have a certain amount of redundancy in the system as well in terms of storage you can remove, aplace where pictures and backup files can reside, and a way for files to be moved around without having to play around with cables or network connections.