"Better" low and mid range power is the most common answer. Better scavenging of the cylinders resulting in a cleaner charge and better burn - and therefore more power. It's basically using "free" exhaust energy from one cylinder to help scavenge the other one.
A 2:2 system such as the aforementioned "Short Shots" or "Drag Pipes" will theoretically give you a better top end at full revs, but will typically have a softer bottom end with a narrow power band and who really rides their bike at full revs all the time apart from real drag racers?
In many ways though you can't just look at the exhaust alone as it needs to be coupled with the right air filter, camshaft and ECM program to get the best results.
Exhaust tuning is really a very complex subject in itself. But one way to look at it is if you look at more or less any kind of "normal" racing exhaust, be it bike, car or whatever they all use a "merged collector" arrangement - i.e. X:Y (and then potentially into Z). These engines all need to be able to produce good power across a fairly wide RPM range. In fact, pretty much any modern car or bike exhaust system uses a merged collector design these days.
If you look at a drag race engine, they typically use individual short pipes per cylinder (X:X) but then they are made to run 100% flat out all the time.
This is a pretty good and irreverent explanation of the physics:
https://rbracing-rsr.com/exhausttech.htm
By the way, these guys make AWESOME pipes. I have one of their pro-stock LSR systems on my Shovelhead and it's a fantastic piece of work. However - I wouldn't recommend buying a pipe from them unless you are prepared to wait a bit and willing to deal with some of their quirks regarding ordering and customer service. If that's OK with you, then go for it!
