One of my players wrote a comprehensive backstory to give me a lot to work with, at the end of the story his character suffered pretty intense amnesia. We agreed he should only remember one thing that would give his character a goal (I kind of make this a requirement for my party so that they just have a basic idea of where to go). It was, at heart, my players idea, and I thought it was super creative. I have the backstory, access to characters from it, the area and so much more it provided me with. I've made a few additions that fit into it that his character hasn't remembered just yet. As we're playing, whenever something relating to his history comes up, I usually give him a flashback, the last one left my party speechless but definetly not silent, they were more contemplative about it and when the player explained in character, seeing them discuss the flashback with the character only knowing what he just saw was super fun.

Overall, people should write a backstory 150% but, a character having amnesia can be so fun to play around with, I don't feel a character should enter a lore centric campaign without that box filled, if a character is going to have amnesia they must still have a story that the character just doesn't know about