Why did the invisible man choose …

The invisible man had no choice. After he escaped from Iping, he was in desperate situation. His important note-books or research journals in which he had written the process of becoming invisible in cipher, had been left behind at Coach and Horses. He badly needed an accomplice. As he was wandering here and there in search of a place to take shelter, he found Mr. Marvel. Fortunately no one was around. So he decided to take his help in getting his journals back from Iping.
Marvel had to comply to Griffin’s wishes because the latter threatened him of dire consequences in case he did not do his wishes. He had already overpowered Marvel by showing him his invisibility. He had hurt him with a rock as well. Thus, poor Marvel had no option left but to comply to Griffin for his own safety.