People wanted a dislike button, got a series of redundant emoticons with very little space for actual disagreement. Which is actually good, because it won't transform Facebook into a mess of flames and negativity. Wait, maybe it's already too late for that...
The purpose of reactions is to provide an alternative for people who don't want to click "Like" on stories of tragedy (e.g. someone's loved one dying, someone announcing they have cancer, or even something non-personal like a news article about a mass shooting) because it sounds inappropriate, like "ha ha, I'm enjoying your tragedy!". There are two reasons why this is important:
1. Some people want to exhibit support but don't know what to say, so they're not comfortable commenting. On more positive stories, they'll just hit "Like", but that would feel inappropriate to do on a more negative story, so they don't do anything even though they want to express their support.
2. Facebook's News Feed algorithm uses Likes to determine how articles are sorted in your News Feed. If people don't Like important stories because they feel the word "Like" is inappropriate, then those stories will be downranked because the algorithm doesn't know the stories are supposed to be important.
Reactions solve both by allowing people to both wordlessly express sympathy for and signal-boost stories that they feel would be inappropriate to "Like".