The main reason is the pure badassness of having the TITAN Z "THE FASTEST GPU EVA MADE! (for gaming)". Actual usefull stuff would be that it allows you to fit a dual-gpu in a smaller mobo and/or smaller case, something like the TITAN Z with a single fan, i would suspect they're going to lower the clocks compared to a standard TITAN Black, though thay might just keep the same clock and make a hot card.
The R9 295x2 does give you better than standard R9 290X clocks, though doesn't any aftermarket cooled R9 290X do that? It also gives you your own waterloop with it, it's basically like buying two R9 290X and put a G10 Kraken mount with a cpu watercooler on top on both.
but overall if you consider this expensive gpus i would just recommend going 780ti SLI, since the TITAN BLACK is a very bad price/performance, and the R9 290X CF is pretty damm hot, the R9 295x2 is a good contender to 780ti if you really want AMD instead, it sits right behind the 780ti in my recommendation.
So i would go GTX780ti if you're considering throwing this much in gpus.
EDIT: and that would be x2 780ti, not x3
EDIT2: and even though it might seem like a bonus that the TITAN Z and R9 295x2 says 12GB & 8GB of vram, it's mirrored to each gpu so you'll still only get 6GB & 4GB of total space for textures.