This perspective is the same as saying we could remove those phrases from English and not have any problems. Obviously, these words have different meanings and connotations.
Almost 100% of the posts at our forum are examples of the significance. We have explicitly described that “appreciate” means “thank you for this post”, as in “this post contributes constructively”, like I appreciate that the post was made. It doesn’t indicate at all that I agree with the post. And this dramatically changes how we engage and how we feel about engagement. If thumb’s up had to imply agreement, then you can’t react with appreciation for a post that expresses an opinion you don’t share. It is so refreshing to feel happy giving a thumb’s up “appreciation” for someone expressing a thoughtful post that I do not agree with. I appreciate that they thoughtfully shared their perspective (which is not a feeling I have about any and every post, I don’t appreciate self-righteous or rambling or off-topic or dismissive posts). So, we give way more appreciations than other forums, and it feels good to do that! It’s so freeing to acknowledge having read and appreciated a post without worry that the reaction will be misinterpreted as agreement. All the higher quantity of appreciations makes for a more public visibility of simple acknowledgement and a friendlier and more supportive community.
So, in this context, hooray is celebrating a post like “we did the release!” as in GitHub’s use of 
which is stronger and different from 
. And “haha” means sharing in some humor, certainly not all posts apply. “totally” as 
is the agreement indicator (again, distinct from appreciation of the post). Sympathies is expressing compassion when people share something troubling. And finally “thank you” as 
got added later when people found they really wanted to express a stronger gratitude (such as for a post saying “I worked all weekend on this, and here’s the progress I made”) distinct from just saying thumb’s-up, thanks for your post. It’s more thanks for your work etc. We felt in general that was just emotionally too strong for generic use all the time and disagree with its use as the default reaction, but we missed having it when we wanted to express something stronger.
And the multiple reactions thing is: I want to express appreciation for a post and sympathies, and sometimes a post has some humor and tragedy at once, and sympathies+haha actually makes sense. And reactions don’t create as much noise as reply posts… I mean, I feel like I get into explaining why there are reactions at all, and that’s not the topic here. Obviously there was a time when forums had no reactions, everyone prefers having them now, the official plugin now supports multiple choices, so there’s consensus about offering different options. The only reason there isn’t consensus about letting people react in multiple ways to the same posts is the same reason there did not used to be choices at all — people not yet recognizing the utility even though it was always there.