I fixed the link.
Partitioning a drive is absolutely necessary if you plan on putting another OS on the same drive for a dual boot (or tri, or quad...). I have used a partition to hold my data, such as music and files, so I could format my OS partition without losing them.
I have partitioned multiple drives to fill different purposes. I have partitioned my main drive to have a dual-boot of two OS's, and then partitioned my other drive to be able to install programs for the two OS's on different partitions as well as a data partition for files that I wanted to be readily available for both.