Fragmentation refers to the condition of a disk in which files are divided into pieces scattered around the disk. 
Fragmentation occurs naturally when you use a disk frequently, creating, deleting, and modifying files.
 At some point, the operating system needs to store parts of a file in noncontiguous clusters.
This is entirely invisible to users, but it can slow down the speed at which data is accessed because the disk drive must search through different parts of the disk to put together a single file.
Fragmentation causes a huge degradation in system performance, and over time can bring your system to a near crawl. 
Fragmentation causes your computer to use excessive resources (memory and CPU time) to complete tasks related to reading and writing files. 
Fragmentation can cause applications to launch more slowly, file access to take longer, system boot or shutdown to slow, slow system hibernation or resume from hibernation, causes videos to drop frames and music to skip.
Optical storage devices use low beam laser to write retrieve data, so there is fragmentation in an optical disk it will be hard for laser to read and write data.