Underneath the user-friendly Windows interface lie complex systems and processes that control the various aspects of the PC. For this reason, from time to time, the PC would require some maintenance work - same as one would do with other complex machines such as automobiles.
Unnecessary processes running in the background tend to use a considerable part of valuable memory, competing with other useful running software such as internet browsers, photo viewing and editing and office suites. When Windows needs more memory than is available to handle all running applications, it uses what is called "virtual memory" - swapping data between the fast solid state RAM memory and the hard disk. When a computer relies heavily on “virtual memory”, apart from considerable slowdown, it is also at the risk of decreasing the lifespan of certain components such as the hard disk.
Apart from an improved user experience, fixing slowdown issues on your PC can also lead to a longer lifespan for your PC due to reduced wear and tear and operation at lower temperatures.