The first parition is VFAT to ensure compatability with Windows - when you put your memory card in a Windows PC it only mounts the first partition and only understands FAT (possibly NTFS too). Creating and using a second partition means you can still use the memory card in your PC to copy music etc. If you only had a single ext2 partition on the card Windows would no longer recognise your card.

Having two partitionts isn't essential, but it does make life a little more convenient.