Why is defragmentation unnecessary?

The underlying filesystems used by Ubuntu, like ext2 and ext3, simply don't need defragmenting because they don't fragment files in the same way as NTFS. There are more details at ext3 - Wikipedia, the free encyclopedia

Some argue that it's actually a myth that we don't need defragmentation. It's argued that we do in fact need it, but only once the filesystem gets pretty full (i.e. less than ~10% free space). Tools are available for defragging such as e2defrag.

Fragmentation is the product of writing files in the first available open blocks on a drive. Over time, as files get created and deleted, small sections of disk open up, which causes newly written files to be split over several such openings. This can reduce performance, although it was much more of a problem in the past with slow hardware and slow disks.

The default filesystem in Ubuntu, ext4 (and until recently, ext3) are designed to limit fragmentation of files as far as possible. When writing files, it tries to keep the blocks used sequential or close together. This renders defragmentation effectively unnecessary.