Do you keep track of your progress by taking placement tests?
I just took one, and I can't believe how much I've progressed by just doing Duolingo mainly, along with other two resources (namely Criminal Case in German, and SuperMemo UX). I'm only midway the German tree by the way, prioritizing strength on each skill.
Here's a screenshot of the result of the test I just took.
And a link for those who also want to try it!
Test your German - Goethe-Institut
I have never taken one, though, I tend to distrust myself... So, whenever I take a test that demonstrates my skills (well, I guess that's all tests), I do worse than what I am actually capable of. Therefore, I probably won't ever take one, but it won't hurt to try once, I guess.
Placement tests are used to see how much you have learned, so you are correct.
I'm not sure exactly what those do because I've never taken one. I'm about halfway through my tree, should I take one? What does it do?
They ask you varied questions from mixed levels to calculate how much you know, and then it gives you an approximate of what level you best fit in. In this case, is goes from A1 to c2. Some questions might be hard to answer even to advanced speakers, but the questions you do answer will give the test an idea of what your level is.
They were mentioned in the original post up there. One is a game; Criminal Case (in German), while the other one is SuperMemo UX Extremes Deutsch, which is like a professional version of Anki that shows you vocabulary while hinging on your perception of how well you remember each word to determine how much time until each word is shown to you again. They're both ways I use to remember vocabulary.