He didn't want to do it, but the execs forced it (no doubt with a giant sack with "$" on the side) anyway:
Ford was, and continues to be, pissed: "When we started shooting it had been tacitly agreed that the version of the film that we had agreed upon was the version without voiceover narration. It was a fucking nightmare. I thought that the film had worked without the narration. But now I was stuck re-creating that narration. And I was obliged to do the voiceovers for people that did not represent the director's interests."
"I went kicking and screaming to the studio to record it," he reportedly said.
And of course Ridley Scott never even imagined it in the film. If you like sci-fi I strongly suggest you watch the 2007 "Final Cut"; it eliminates the narration and undoes some other edits, and restores the movie to its rightful place as one of the best futuristic dystopian films ever.