Here's a bit from Chapter 1, where Ralph has just been voted in as chief by the other boys:
“I’m chief then.”
The circle of boys broke into applause. Even the choir applauded; and the freckles on Jack’s face disappeared under a blush of mortification. He started up, then changed his mind and sat down again while the air rang. Ralph looked at him, eager to offer something.
“The choir belongs to you, of course.”
To allow Jack to be in charge of the choir (who then become the hunters) is not a decision Ralph makes tactically, but one he makes out of pity, because he sees that Jack is embarrassed not to be chosen as chief, and because he wants to "offer... something".
What that does is allow Jack, who Golding has already underlined as the obvious leader from the group, a whole group of boys under his control, sowing the seeds for a rebellion later. Ralph too seems slightly in awe of Jack, and perhaps it would have been a better move, rather than splitting up the choir and the other boys, for the two boys to rule jointly as chief.
On the other hand, Ralph is aware politically that Jack needs to feel in control of something - and perhaps the choir is a compromise that Jack would have insisted upon were he not offered it. It's difficult to tell - how do you control someone like Jack? How do you make sure they don't just shout "Bollocks to teh rules" and run off to anarchy?