The disk drive is the bottleneck in any computer system for one simple reason: Information can only be processed as fast as it can be delivered. Even the fastest disk drives and disk arrays cannot deliver data anywhere near as fast as RAM can store it and the CPU can process it. From that, we can derive a simple fact: Anything that slows down the disk storage system will slow down the entire computer.
That's why disk fragmentation is such a huge barrier to system performance. Reading or writing a file that's broken up into several pieces is like trying to make a sandwich with the bread in the kitchen, the meat in the attic, and the condiments out in the garage. (Worse, actually -- files can be fragmented into hundreds or even thousands of pieces.) The drive must gather the bits of the file from multiple locations. Meanwhile, the processor and RAM just sit there, twiddling their electronic thumbs and waiting for the data to be delivered.
The key to computer speed, then, is to handle fragmentation early enough that it can't make a significant dent in performance, which is why an automatic defragmenter is vital for each and every system. With defragmentation occurring automatically in the background, one of the major barriers to computer speed is totally eliminated.
Simply put, without automatic defrag software in your corner, you may find your servers and workstations soon laying facedown on the mat.