I am not sure of all the reasons why, and there are probably various reasons.
One reason I have been told is that it helps minimize potential data loss. Sometimes an individual partition can become damaged. If everything is broken up into lots of partitions, you won't lose that much.
Another reason is if you use disk quotas. Since quotas are implemented against partitions, you can become much more granular the more partitions you have.
It's too early in the morning for me to remember any other *good* reasons, but if I run across more I will let you know.