It's a welcomed career movie for Halle (supporting franchise role). When she was first announced it said she'd be head of the CIA which sounded quite a boring authority role, but the fact she's head of the US version of Kingsman means she could have some cool stuff to do.
She'll have Matthew Vaughn to thank. Halle nearly worked with him twice before (X3 and DoFP). I imagine they liked the idea of a woman of colour being the head for diversity reasons.
Also props to Vaughn for hiring older actresses. Halle is 50 this year and Moore is 55.