Oh, I see now where it's being put. But what purpose does it have? I mean, you can always manually change the keyframe position.

It's useful when you need to apply typical animation (such as zoom or pan) to many slides. These kinds of animation work best when the second keyframe is set at the end of the slide. Thus having set the 2nd keyframe position to 100% you don't need to adjust it each time, saving some time and effort

Hi. I've been messing with settings lately, and have found the "default position of the second keyframe" setting. But what does it exactly do? I can't see any difference changing it.

This setting adjusts the position where the second keyframe will appear when you add it. To check that out, open up the slide editor, select any layer having only one keyframe and click the Animation tab. Now click Add keyframe, and look at the timeline. This very position where the keyframe appeared is called the "default position". Hope that helps