Mediatek has hyped up its octo-core chip a lot of over the past few months, in anticipation of its arrival. As devices with their 8-core chips are starting to ship, they want to show that the criticism against 8-core chips being "unnecessary" is at least somewhat unwarranted, by comparing how Gameloft's new Modern Combat 5 game looks on an 8-core device against a quad-core one.
The demo shows that the game gained a few extra visual effects on the 8-core device compared to the generic quad-core smartphone that Mediatek has chosen to use. Still, it's not very clear how true this comparison is, and if everything else is equal, except for the CPU having twice as many cores.
We still have to ask whether the 8-core chip also comes with a more powerful GPU, or whether the game was optimized to take advantage of that new GPU more than it does on the quad-core device. We've seen in the past how some games looked better on Tegra 3 compared to the same game on a quad-core Exynos or Snapdragon processor, because the developers optimized them for Tegra 3.
That being said, if 8-cores are going to go mainstream, games and browsers are going to be the first apps to take advantage of such parallelism. In games, it's just a matter of developers supporting that many cores/threads, from the beginning (which is why you'll need to wait for new games supporting 8-cores, which could take a while). With the next-gen consoles supporting 8 cores now, and many laptops/PC's using quad-core Intel chips with 8 threads, the gaming industry should be among the first to take advantage of 8-cores, though.
The browsers, or at least Chrome, has been heavily multi-threaded for a long time, so they can probably already take advantage of 8-core chips from the moment they come to market. However, other than the browsers right now, and games (slowly) starting to support 8-cores, there won't be many other apps supporting 8-cores anytime soon, until a huge number of devices in the market already have 8-core chips, and until it gets much easier for developers to take advantage of this many cores even in normal apps.