Defragmentation increases the speed of data access by reorganizing associated data files stored on a disk to occupy contiguous storage locations. Defragmenting a disk minimizes head movement, which reduces the time it takes to read and write files to the disk. Over time, a traditional mechanical hard drive becomes fragmented when there is not enough free space to fit a complete set of new data into a contiguous memory location. The computer therefore stores this data in pieces scattered throughout the hard drive. As the data is now dispersed, accessing it will obviously take more time and effort. This reduces hard drive performance. To remedy this problem, defragmentation is performed on the disk.
Defragmentation will perform numerous data rearrangements such that all the scattered data is brought together and stored in a contiguous memory location as efficiently as possible. This organized rearrangement of data will make data easier to access. Now the hard drive no longer needs to work harder to access a particular set of data. This will improve performance.
Note that defragmentation is entirely related to efficient head access to data and efficient storage of data on the (spinning) platter. Defragmentation reduces latency caused by head and platter movement when accessing data. This can therefore only be done on a mechanical hard drive. SSDs don't have these moving parts, so they don't have latency caused by them.
SSDs have a life cycle, also called a program/erase cycle. You can perform a specified number of writes and erases to the SSD before it starts to become inefficient. This life cycle is measured in terms of total bytes written (TBW), mean time between failures (MTBF), etc. Since defragmentation deals with a lot of data written, moved, erased and reorganized, SSD defragmentation will directly affect (degrade) its life cycle.

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