Kim Tripp explains partitioning:
So, you’d still need to determine if this is the right approach. But, the main point – partitioning really isn’t designed to give incredible gains to your queries. It’s meant to be better for data management and maintenance. However, some partitioning designs can lead to query performance benefits too.
This is a nice introduction and makes a good point: performance benefits to partitioning are incidental to the real benefit, which is simplicity of administration.