For finales perhaps its provide closure and give a sense of completion. You know how movies and fictional TV shows often have endings. They are done with that particular situation or story-line and they wanna move on. Reality show series (like Love and Hip Hop/ Real Housewives, etc.) are like regular fiction based shows. They need a series of dramas and resolutions to continue the story. As the story continues through each season, the old drama is still there but it isn't fresh anymore and need something in its place. Even so, reality shows doesn't have much of a plot but more like a theme (like love, relationships, friendships, etc.) and an angle (hip hop industry, society issues, bad behavior in youth). By the way it makes it easier for fans/viewers to move on to reunions and new seasons
For reunions: Mostly for ratings, but also perhaps for the fans.... Perhaps for the contestants/cast members to clear up things, explain the situation more from a fresh and more rational perspective and approach, to see where the contestants/cast members stand. This because filming is filmed months before and perhaps they don't feel the same way anymore or something has change.
Drama starts right back up because they are reliving it when they keep talking and hearing about it. Imagine if something happened three months ago that made you upset wouldn't you be first angered/annoyed/etc. then start thinking about it it will hurt/piss you off, which turns to DRAMA).