What are the benefits of having more cores?

More cores is sort of like trying to compare core speed. It can be done, but should be between similar cpu's to be meaningful. The more cores/threads, the more data that can be processed at the same time. Using the i7 as an example, hyperthreading allows 8 threads to be scheduled and ready for the cpu cores but there are still only 4 physical cores processing the data. It's a little more efficient but that's why ht enabled quad cores don't process like 8 core cpu's like the 5960x and why non ht quad core i5's with 4 true cores process more than dual core i3's with ht that also process 4 threads.

Aside from that, it's hard to compare amd 8 core to intel quad core or old intel core 2 quads to current i series quads. Architecture has changed, cpu core speeds have changed and it with improved efficiency they're processing more per cycle than say an older version. For example intel, a 7yr old cpu at 3ghz doesn't process the same amount of data as a current cpu at 3ghz. An amd cpu at 3ghz doesn't process as much as a current intel cpu at 3ghz.

Amd's fx series does have more cores, but they're much weaker cores. Even in most multitasking situations a quad core i5 can still process more than an 8 core amd purely because the cores of the intel are processing the data that much faster. There's no easy way to say 'editing' or 'gaming' use. 5 different editing programs and 5 different games can perform very differently and trying to lump them together as the same thing is hard to do. In gaming, even a dual core ht enabled i3 is capable of running most games within 1-2fps of the 8 core fx 8350.

Synthetic benchmarks can make it just as confusing because they're designed to take advantage of every capability a cpu has which in a perfect world would be accurate. Real programs people use on their pc's don't work quite the same. Much the way ram testing with synthetic benchmarks shows a large difference between two different ram sticks with different specs or between single and dual channel memory operations yet when tested with real programs there's nearly no difference. Take gaming for example, an fx cpu might have a really high synthetic benchmark score compared to a quad core intel or dual core/ht i3 - but then when tested using actual games, the quad intels beat the fx and the i3's match them.

The best way to get a feel for the benefits you'll get from each cpu is to look for real world tests that focus on similar workloads you might be doing. If possible, try to find results for specific programs you'll be using. If it's gaming and you want to play witcher 3 but not gtav, look for witcher 3 cpu benchmarks. If it's video editing, and you use handbrake then look for handbrake benchmarks. Handbrake and sony vegas and adobe premiere are all used to edit video, but all 3 perform differently on different hardware. Vegas seems to run better on amd/ati gpu's, premiere on nvidia gpu's and handbrake can make use of intel's igpu in the form of quicksync. Cpu performance alone on all 3 can vary as well. It's never quite as simple as 'intel gaming' or 'amd editing'.