When Android was new, you could easily put what you wanted on your SD card. Pictures, apps, data, etc.
Progressing through Android 4, 5, and 6 (what I am familiar with), Google implemented various seemingly antifeatures to make things difficult for a user who wants to use the SD card for main, but still removable, storage.
What are the stated reasons behind this? What problems are they trying to solve? Why was this considered a good solution?
A great answer will have technical and nontechnical explanations. For example, how do you explain to Aunt Suzie that she can't use that shiny 200GB SD Card she got on sale from Amazon?