How can I separate processing input and update?

I know that game loop is broken up into three distinct phases: processing inputs, update, and render, but I just can’t see how I can make processing input and update independent of each other.

Let’s say I have a button. If I click on the button, it will increment the number by one on the screen. How can I make this possible? Is it OK to update the logic while processing the input?

For example:

 while (!ended)
 {
   while (SDL_PollEvent(&event))
   {
       IF MOUSE EVENT
          IF USER CLICKED ON THE BUTTON
              NUMBER += 1;
   }

   update(); // no job for update as number was incremented above
   draw();
 }

Why would I need a distinct update function if we could just update logic/values while processing input like the code above?

Please show me an example using SDL poll event system.

 while (!ended)
 {
   while (SDL_PollEvent(&event))
   {

   }

   update();
   draw();
 }