I used the cloak of undying loyalty on a goblin. then when i used soul…

I used the cloak of undying loyalty on a goblin. then when i used soul steal(the skill obtained when u equip the cloak of... to lars)a message appeared telling that "Lars has stolen the monster's soul".
and then, when i opened the menu,(Esc) i saw a label "creatures" on the lower right of the screen and the goblin that i captured was there. What is the use of capturing a monster with the cloak?How to use it?