Short answer, radius arms primary function is to correct caster and (sometimes) driveline angle. But they can also provide a little additional flex in one of the following two ways:
1) Delta arms have a narrower profile at the axle side, which allows more articulation. With the stock arms, there is some articulation available by squishing the bushings, but at a certain point, the arm itself binds against the brackets on the axle. The Delta arms are a little narrower than stock to reduce this binding effect. It is also possible to grind down your stock arms at the axle end to narrow the bushing mount area to achieve the same thing.
2) Superflex arms relocate the mounting points on the axle side to allow for more flex without binding.
You can also get a ton of flex hillbilly style with the hitch pin mod.
Aftermarket arms have other benefits, like pushing the axle forward (if desired), and correcting caster without drilling any holes in your axle mount brackets, or welding anything onto the underside of the truck.
Whether any of these solutions are worth your money is totally up to you. If your only goal is more articulation, you might have better luck with a spring with longer free length and longer shocks.