Virtual Memory

Why is it possible to load some page to memory while keeping others on disk?

1. Programs often contain code for handling errors (e.g., using try-catch blocks) that occur rarely. During the execution of a program, if such a rare error doesn't occur, it is not necessary to keep the error-handling instructions in memory until the error happens.
2. When an array (or another data structure that serves like a container) is created, many times a very large size is allocated for the array, just in case the worst case occurs for a certain algorithm. As long as the worst case doesn't occur, we can store only the used-up potion of the array in memory.
3. Many functions/methods in a program are rarely used, so it is not necessary to keep the instructions of such functions in memory until the program calls the function(s).

In other words, there are many instances in which we could choose to keep a certain program page on disk rather than loading it to memory.