This is what my friend told me when I learned how to Dj.
A straight arm is used mostly for Dj's that scratch more than mix because the straight arm keeps the needle in the groove so when you scratch, the needle/tonearm doesn't skip or hop. You can be more aggressive at scratching with this kind of arm.
An S Curved tonearm is ideal for dj's that do more mixing than scratching because it puts the needle so it curves with the record. If you try to scratch sometimes with an S Curved Tonearm it usually stays in the groove but you have to scratch alittle lighter than you would with a straight because it hops. An S Curved can be setup like a straight arm if the proper weight and tone arm height is applied.
Thats all that I know about this and i'm sure there is a more detailed reason why but if my info is messed up, feel free to correct me. hope this helps.