Answer:
Because he needed to protect Percy and camouflage himself within New York society. This has nothing to do with the camp, but with the relevance Percy has within the magical universe where the Greek gods live.
Step-by-step explanation:
Although you did not present the book to which this question refers, we can see that you are referring to Percy Jackson.
Percy Jackson is a young New Yorker who doesn't know he is the son of Poseidon, the Greek god of the seas. Percy does not know this, because he needed to be protected, as he is the son of one of the main Greek gods and the Olympians wanted to kill him because he thought he was very powerful.
In addition to hiding the truth from Percy, Poseidon appointed Grover as Percy's official guardian. Groover is a satyr and could not present his true form within New York society without revealing his true identity and Percy's identity, so he disguised himself in order to accompany Percy in New York and protect him.