Hidden interface controls that affect usability

I get why you would hide interface elements to use the screen real estate for something else.

I have no idea why some interfaces hide elements hide and leave the space they'd taken up unused.

IntelliJ does this, for example, with the icons above the project tree. There is this little target disc that moves the selection in the project tree to the file currently open in the active editor tab. You have to know the secret spot on the screen where it is hidden and if you move your mouse pointer to the void there, it magically appears.

Why? What is the rationale behind going out of your way to implement something like this?

Some people complain about "visual clutter". Too many stimuli in the field of view assault their attention, and ruin their concentration. Such people want everything that's not in the focus of attention be gone, or at least be inconspicuous.

Some people are like airliner pilots. They enjoy every indicator to be readily visible, and every control to be easily within reach. They can effortlessly switch their focus.

Of course, there is a full range between these extremes.

The default IDE configuration has to do a balancing act, trying to appeal to very different tastes. It's inevitably a compromise.

Some tools have explicit switches: "no distractions mode", "expert mode", etc, which offer pre-configured levels of detail.

This is a good idea. In basic/beginner mode, every control should be readily visible and discoverable.