It was important that he use the bonding ritual spell to create a connection between him and the plant because he didn't raise the plant himself and isn't familiar with it.
The two of them are strangers, so they need a good basis for a pleasant cooperative relationship. This was why he didn't waste time about it before doing it.
He also used his Beast Link spell on the green-faced plant so that the two of them would be able to communicate and locate each other easily.
With this done, he took the green face plant outside the sect to feed it. His main goal is to rob and kill for money. But he took the green face plant with him so that he could feed it the corpses of the people that he kills.
This is something that he wouldn't have been able to do with the blood vine. He would have had to plant the blood vine in his floating island, and it would have needed to stay there forever and ever.
