Partitions are useful for a couple things:

1) Letting you install multiple operating systems without them affecting each other. This only matters if you want to run say Vista and Windows XP and Linux on the same computer.

2) Letting an older operating system work. Some operating systems won't be able to use disk partitions larger than a certain size, e.g. 4 GB.

3) Organizing your files, e.g. always putting your music on drive d: You can do the same thing with folders / directories though.

For me partitions are a pain because I have to think about which partition to put stuff on: how much space left on this one? on this one? IMO it's far better to use a single partition unless you know for sure you have a need for multiple partitions.