Q:

Using delta time vs not using it

I've started learning libGDX and went not very deep for now.  In one of the tutorials I have noticed that people use deltatime which is (if I understood it correctly) time between 2 frames which should be around 0.016 (60 fps).  Then I saw a person stating that it's a bad practice to use deltatime as it is quite difficult to implement the right way. 
Here are two questions I have about the deltatime

When do you want to use it and when do you want to use a fixed number? 
I also suppose that deltatime is a wrong thing to use in multiplayer network games because people may get different fps and the calculation will not work as intended.  Is it a valid statement or am I wrong? 

A:

In the event of differing FPS or uncapped FPS (no vsync), you may still want different aspects of the game to run at the same speed.  For example, you probably don't want a player moving around at twice the speed in they are getting 120 FPS while a player getting 60 FPS walks around at normal speed. 
moveSpeed= baseSpeed * delta;

The higher a person's frame frame, the lower the delay between frames so your can multiply a base speed by the delta to get a scaled movement speed.