What does 'Event.Use()' actually do, and when to call it?

1) What does 'Use' actually do?

Consider the code snippet below:

 void Processing()
 {
     bool resourceConsumed = false;
     
     // Process #1
     if(!resourceConsumed)
     {
         // Consume resource for process #1
         resourceConsumed = true;    // This is equivalent to Event.Use()
     }
     
     // Process #2
     if(!resourceConsumed)
     {
         // Consume resource for process #2
         resourceConsumed = true;    // This is equivalent to Event.Use()
     }
     
     // Process #3
     if(!resourceConsumed)
     {
         // Consume resource for process #3
         resourceConsumed = true;    // This is equivalent to Event.Use()
     }
     
 }

In above case, the moment we consume the resource and set it to be consumed, following processes will not perform their processing since no resource is available to them. But even if we consume the resource and do not set resourceConsumed then following process can still consume the resource.

Now setting resourceConsumed = true does in above case to true is what Event.Use() does in case of Event. It just sets that event as used so that no other GUI element will process it. But if you do not set it to be used then other GUI elements can still access that event and use it for their processing.

2) When and why would I consider calling it?

Now in your code snippet

 var e = Event.current;
 if (e.type == EventType.MouseDown && e.button == 0)
     // do LMB stuff

if you use Event.Use() after processing LMB no other GUI element will process that input like a button will not get clicked and so on. I know it might sound useless with this example but there might be cases like when two buttons are overlapping and you don't want both of them to get clicked and only one at the top to do so.

Now in another example of yours.

 if (e.type == EventType.MouseDrag && rect.Contains(e.mousePosition))
 {
     rect.x += e.delta.x;
     rect.y += e.delta.y;
 }

If you are moving the window which is over some other GUI element, say another node Window then not using Event.Use() in your code snippet will cause both these GUI elements to process that input which I believe is undesired. But if you call the e.Use() at the end of your if block above then only this window will process this input (if it is the first one to receive that event and event is still available for it to process), which I guess is desired effect.

Recently while developing a quest system based off Node Editor I used navigation keys for panning the window containing nodes. Now in my case I did not want any GUI element to process the navigation keys since I wanted to use it to pan so after performing the panning action I just used Event.Use() to avoid certain glitches that were causing because other GUI elements trying to process those keys, which wan unintended for me.

Hope this clears things for you.

Note: I have tried my best to steer clear of the prohibited words and phrases. ;-)