A drive is partitioned and formatted in order to make it usable for the OS you are running as well as seeing one or more new logical drives. In order to install the OS or simply copy and store any type of file on a drive this has to be done first.
Partitions see different types as well. Windows runs on an MS primary type partition while the open source OS Linux depending on which distribution can also run on the extended type. A maximum of 4 primaries can be seen on a drive while any more have to be extended.