I wouldn't say they're necessary but I think conflicts and disagreements are a good way to just release tension and resentment so you can be honest about whatever you may be feeling. Obviously there's a point where it can get counterproductive and people say things they don't mean in the heat of the moment, but that aside I think they can actually benefit communication. Being pleasant and agreeable all the time to avoid conflict can actually be very emotionally detrimental, so a healthy skirmish every once in a while might be good in the long run.