For a basic system seeing only one OS one drive is all it takes simply to see the OS installed on it as the main boot device. The type of build and plans to dual or multiboot different operating systems or simply having a second for storage and backing things up is one main reason why people will often go for a second drive internal or external.
Laptop/notebook users may go out and buy an external usb drive since those are compact and the one optical drive would need to be sacrificed in order to see a second hard drive added internally. For a single large capacity drive you have the option of seeing have one single large primary partition or see two smaller partitions one for the OS and the second for storage.