Raids and Dungeons in old content

Hey,

I am pretty new to wow and thought I would try to replay the old expansions, because I am pretty lore focused, starting from cata. I think the chromie time features works pretty well, but I can’t figure out the dungeons/raids. I tried doing one of the zone dungeons in deepholm but couldn’t find a group. Are dungeons/raids necessary to progress or understand the story? For which expansion would I still find raid groups?