Partitions are created, on a mechanical hard drive, from the outside (faster) part to the inner (slower) part of the platters so partitioning makes it easier to sort your stuff to areas based on speed. for example making 3 partitions would give you C:, D:, and E: which correlate to transfer speed: fastest, medium, and slowest. So you can install games and programs needing faster access to C:; Pics, Movies, Music... to E:
What you really want to avoid is having to read and write to the drive at the same time. Thats where SSD's really shine since they don't waste 9ms or more moving the heads like HDD's do.