Personally, I think it is a good policy for work culture. 20% does not just mean you have to work on your own idea but you can work on your other passions or to

Personally, I think it is a good policy for work culture. 20% does not just mean you have to work on your own idea but you can work on your other passions or to develop your skills. Say you are a backend dev and are passionate about ML, you could do a 20% with another team which is working on ML and develop your skills or just test waters. If that work has impact you can also include it in your perf.