Here's why:
* They're guarding the entrance to the cave. The Grunts are guarding the entrance to the cave that leads to Lake Acuity and the legendary Pokemon, Azelf, Uxie, and Mesprit.
* You need the Galactic Key. This key is essential to open the locked gate that the Grunts are protecting. You obtain the Galactic Key later in the game, after completing a significant storyline portion.
Here's how to get the Galactic Key:
1. Defeat Team Galactic in Veilstone City: You'll need to fight Team Galactic at the Veilstone City Gym, taking down their leader, Jupiter.
2. Go to the Galactic HQ: After defeating Jupiter, head to the Team Galactic HQ in Veilstone City. You'll need to go through the building, battling more Grunts along the way.
3. Find and defeat Mars: In the HQ, you'll encounter and battle Mars.
4. Retrieve the Galactic Key: After defeating Mars, you'll obtain the Galactic Key, which is required to unlock the gate in Celestic Town.
Once you have the Galactic Key, you can return to Celestic Town, open the gate, and battle the Grunts to continue your journey.