I've asked the related question in security SE: Why does not updating a MacOS keychain password cause applications to become troublesome? and the answer there explains that keychain is used by various applications to store "sensitive data such as credentials, certificates and the like... passwords..." and goes on to explain that a total list of all possibilities is beyond the scope of the question.
I changed my password by using recovery mode. I'd changed it yesterday, then it suddenly appeared to stop working.
As I continued, I received the message (photo of screen shown below):
The system was unable to unlock your login keychain.
Since I don't use my keychain at all, and since I did not understand the implications of each choice, I just selected
Once I was using my computer again, I have a persisting sequence of dialogue boxes that will not go away. They seem to interfere strongly, rendering other applications unresponsive (beachballing) until I cycle through them once or twice by clicking
Cancel three or six times. Preview is particularly difficult to use now.
For example, I have found that when I open multiple images in Preview, I can move easily between them through cursor operations on the sidebar, but as soon as I try to use the arrow keys preview freezes and beachballs until I click through the keychain dialogues again. Same for other menu items like close and save.
I have similar behavior for other apps. For example when TextEdit has an .rtf document that contains links, trying to use hotkeys to save will also trigger a dialogue box, but I'll ask primarily about Preview to keep it simple.
QUESTION: Why specifically would Preview access the keychain when I use keyboard shortcuts, but not cursor clicks or menu items for the same operations? Any thoughts why this (sometimes) happens with TextEdit as well?