"Optional" objective in quest matter?

Hi, while playing I noticed many quests have some "Optional" goal which we can do, like finding x clues, talk to x people. Now, some of them I accidentally didn't do because once I wandered into the main objective, it just change to cutscene and move on to the next part of the quest. Not able to go back. My question is, do these quests matter? I'm somewhat of a completionist, so I'm not sure if I miss anything. Is there a possibility that the quests can be redone in the future?