Polar is basically a personal knowledge repository.
The idea is that you use Polar for all of your education. Either official education or continued learning.
Every textbook or technical paper should go into Polar.
You can then annotate the textbooks directly including text highlights, comments, etc.
It supports spaced repetition systems like Anki so you can continually review what you've learned and NEVER forget anything.
It doesn't make sense to pursue a PhD or spend 6-12 months researching a topic to just forget 80% of it due to bit rot.
The reason I named it Polar is that it's meant to 'freeze' all of your knowledge so that you never forget it - ever.