A few reason... For me, I find it very pointless and hate how many companies are partitioning large drives. My father's computer, for example, has it's C: nearly full and it's D: nearly empty
Anyway...
Data safety: Store your data on one partition and windows on another could help should you have any type of major software crash/failure with your Windows partition. Obviously, if the whole drive were to die, both/all partitions would be lost.
Mutliple operating systems: Seeing as it works as two drives, you can easily install an operating system on each partition without them conflicting with each other.
Backup/Tools: Many times computers have a recovery partition or tools. This way they are not required to include disks for all the software and recovery can be made into a one-step process.
Clutter: If you know you'll be adding/removing a fair amount of data, partitioning may be a good idea. For example, partition a large drive to have 25% for Windows and software and the remaining 75% for capturing/storing videos. This won't leave large fragmented gaps in your data as items are added or removed.