What are your thoughts on annotating books?

Book in picture - Tennessee Williams’ Cat on a Hot Tin Roof

I was a believer of keeping my books in pristine condition, no stray marks, no dog ears, nothing. But in the last few years i’ve started annotating my books and have found it really helpful, from keeping track of difficult to understand texts or annotating books and exchanging them with friends. It has honestly made reading a more memorable experience for me.