The biggest effect that arm sweep achieves is how it creates suspension bind. The more angled a hinge pin is, the more suspension bind you will create as forces work against each other when compressing.
Arm sweep essentially makes the front end stiffer because the hinge pin is essentially angled towards the opposite direction that the outside arm is trying to hinge in compression in a corner.
Like most set-up changes, when stiffening the front end of a car a harder front end will delay weight transfer and create less grip going into a corner. However, as the car starts to turn and lean the increased rate on the tire will create more pressure on the tire essentially creating more traction, hence more middle and exit steering.
I use arm sweep in everything. To me it's a default set up thing. I feel it makes the car easier to drive going into the corner. I am not saying this is the best approach, but most of the time it works. Only if I am looking for more initial reaction will I take it out, however most of the time a little more camber, shock angle change or an adjustment on the inner camber link can get me the reaction I want.