Confused about Obito's ability So Obito can transfer any of his body parts that are in contact with the enemy to the time-space of the Kamui and make it look like he cannot be touched. 

But then in some of the fight scenes, for instance I remember when he was fighting Guy and Naruto, he was about to touch Naruto but guy kicked him from behind so he didn't solidify and let himself pass through Naruto and let Guy's kick pass through him. Well at that time, why did he even need to transport his hand too? Couldn't he just transport the part of his body that was about to be kicked by Guy and then touch Naruto?

Why did he transport his fingers using the Kamui too? It was completely unnecessary and he could have taken down Naruto if he didn't do that..
Obito/ Tobi's kamui is his mangekyo sharingan ability unlocked presumably when he saw Rin die. It is a space-time ninjutsu that allows him to transfer objects into his own pocket dimension, including parts of his own body. While Obito is transfering anything between regular space-time and his own space-time his object not being transfered is vunerable