One of the reasons why Su Mo had taken in An Jian was because he needed An Jian to find out where Gongcheng Taihao was.
After all, Gongcheng Taihao wasn't in Sky Tyrant Tribe and he might not come back soon. Su Mo could not wait for him forever.
As such, the Dark Shadow Tribe was a good helping hand for Su Mo.
An Jian was the top talent in Dark Shadow Tribe, whose strength was almost equivalent to an Empty God's. He was a top figure in the Dark Shadow Tribe and he could use the strength of the Dark Shadow Tribe.
He believed that a skinny camel was stronger than a horse. Although the Dark Shadow Tribe no longer had the glory of the ancient times, its root power was something other ordinary tribes could not compare to.
Another reason why Su Mo had taken in An Jian was for the Firmament Sect.