There are three audiences for a reaction:
- the author of the original note,
- the creator of the reaction,
- other readers observing both.
If you eliminate the author from seeing the reaction, there are still the other two. The creator may want to know later which posts they liked and when. The creator may want to search through their old liked content to find something they half remember.
The other readers may want to see aggregate counts of reactions of different types. Perhaps they want to search for the mutually-liked posts of the people they follow. That could be a strong signal.
Could these be handled in another way? Certainly. But what would be the benefit of inventing a different mechanism?