Better flow at higher boost pressures.
As noted less pressure loss to expanding hoses.
Looks are just a side benefit.
There is a school of thought that hard pipes improve lag, or decrease the time needed to fully fill the IC system. I think with larger turbos, this train of thought is overrated as the total volume difference due to hose expansion is pretty dang small compared to the total volume your turbo is putting out.
The plastic stock ones can be restrictive. (I gained 2 psi just from removing the lower alloy elbow, and the upper plastic "flat pipe". No other mods. And I have to say, along with the 2psi of boost, it made the car eager to run, and more responsive. (both of the pipes I replaced don't swell up, or expand under boost, but they did restrict flow.)
Improved flow is the biggest thing, as adjuster just noted. If you look at your stock pipes there's a lot of inconsistant pipe sizes and some of the angles are accordian flex stuff...these really hinder flow.