Why?
Because this gives the Operating System a way to securely manage memory.
Why is secure memory management necessary?
Imagine if there was no logical addressing. All processes were given direct access to physical addresses. A multi-process OS runs several different programs simultaneously. Imagine you are editing an important letter in MS Word while listening to music on YouTube on a very recently released browser. The browser is buggy and writes bogus values to a range of physical addresses that were being used by the Word program to store the edits of your letter. All of that information is corrupt!
Highly undesirable situation.
How can the OS prevent this?
Maintain a mapping of physical addresses allocated to each process and make sure one process cannot access the memory allocated to another process!
Clearly, having actual physical addresses exposed to programs is not a good idea. Since memory is then handled totally by the OS, we need an abstraction that we can provide to processes with a simple API that would make it seem that the process was dealing with physical memory, but all allocations would actually be handled by the OS.
Here comes virtual memory!

Why?
Because this gives the Operating System a way to securely manage memory.
Why is secure memory management necessary?
Imagine if there was no logical addressing. All processes were given direct access to physical addresses. A multi-process OS runs several different programs simultaneously. Imagine you are editing an important letter in MS Word while listening to music on YouTube on a very recently released browser. The browser is buggy and writes bogus values to a range of physical addresses that were being used by the Word program to store the edits of your letter. All of that information is corrupt!
Highly undesirable situation.
How can the OS prevent this?
Maintain a mapping of physical addresses allocated to each process and make sure one process cannot access the memory allocated to another process!

Clearly, having actual physical addresses exposed to programs is not a good idea. Since memory is then handled totally by the OS, we need an abstraction that we can provide to processes with a simple API that would make it seem that the process was dealing with physical memory, but all allocations would actually be handled by the OS.
Here comes virtual memory!