Why locals love it
The Tallinn Old Town is actually not that big, but that doesn't mean you can't get lost. The Toompea hill, especially, is well known for leading people through twists and turns into dead ends. The Kohtu platform is ideal if you wanna see the whole of Tallinn without having to climb the narrow stairs to a certain very tall church in the Old Town.
Why you should visit it
Once you've reached the high ground and have actually found the platform, you'll witness a sea of odd-looking red rooftops, grey guard towers and beautiful spires, as well as the modern Tallinn skyline in the distance. If you can't see the highrises, then it means you're on the wrong platform. But keep trying, you'll get there. Eventually.
Don't even think about visiting it during the day in summer, because it's flooded with cruise passengers who all leave by about 3pm.