- you can format partitions independently
- you can mount, unmount partititions (mostly in Unix environment)
- you can use different file systems for different partitions
- easier recovery
- you can use encryption for certain partition.
It's always good idea to use one partition for OS (system files) another for data etc.
In unix/linux systems there is also so called swap partition, it's used for virtual drive (in case when there is not enough RAM)
In case you have large external Hard Drive you can use LVM if you familiar. Or just one huge partition, why not?