When do I need to call mainloop in a Tkinter application?

Compare a program with an interactive GUI to a program that calculates the hundredth Fibonacci number. All the latter program has to go through a series of steps in order, top to bottom. The set of steps and their sequencing can be known in advance, and it'll remain constant no matter how many times you run the program.

But the GUI program is different: at any given moment, it has to be able to handle all sorts of different kinds of events and interactions. This requirement is often implemented using a programming construct called an event loop. An event loop is the central control structure of a program. It waits for an event to happen, and then dispatches the appropriate handler.

You didn't mention which interactive shell you're using, but I'm guessing it's IDLE. IDLE itself is a Tkinter program, and it already has an event loop going. So possibly the Tkinter code you are typing into the shell is getting bound to IDLE's event loop.

When you execute your code, the tkinter window will refuse to open without there being a mainloop function.

For example this will not work:

from tkinter import*
root=Tk()

This, however, will work:

from tkinter import*
root=Tk()
root.mainloop()

The answer to your main question is, you must call mainloop once and only once, when you are ready for your application to run.

mainloop is not much more than an infinite loop that looks roughly like this (those aren't the actual names of the methods, the names merely serve to illustrate the point):

while True:
    event=wait_for_event()
    event.process()
    if main_window_has_been_destroyed(): 
        break

In this context, "event" means both the user interactions (mouse clicks, key presses, etc) and requests from the toolkit or the OS/window manager to draw or redraw a widget. If that loop isn't running, the events don't get processed. If the events don't get processed, nothing will appear on the screen and your program will likely exit unless you have your own infinite loop running.

So, why don't you need to call this interactively? That's just a convenience, because otherwise it would be impossible to enter any commands once you call mainloop since mainloop runs until the main window is destroyed.