What is the purpose of the Keaton grass in Majora's Mask prologue [closed]

Part of the design philosophy (or so it appears to be) of Ocarina of Time and Majoras Mask is to show you how the world works and drop hints via NPCs, rather than explicitly explaining what each mask does and how it can be used. A few NPCs in the game will mention something vague about why the plants move around like that, but the player needs to know that these particular circles are different and significant and they need to figure out the interactions for themselves. This is part of the exploration aspect of the game. Exploration is not limited to world-space, but can extend to game mechanics and story.

The purpose of the grass moving even when you can't do anything with it is to teach you about game elements without telling you what they are. How would you know to try the keaton mask there if it hadn't moved before? The world needs to be consistent, but slightly mysterious. You cannot explore a world you know everything about. The game says "Here is the world", and the world never changes unless you change it. So, the grass moves when you cut it? "Why?", you ask. That drives exploration. It encourages you to solve the mystery.

Part of this is the concept of "gating" in game design: you show the player a lock and that they want to get beyond that lock. The player then, through their own action, opens the lock with some "key", and gains access to the rewards. The grass is the lock, the Keaton mask is the key, and the Keaton himself as well as the quiz is the reward (which leads to a piece of heart).