Mainly for making music, some plugins that are doing simulations are very computationally expensive, so it is not possible to run many instances in realtime using typical 4-core PC. So multicore cpu like in my case helps a lot with this. Another thing is that I use a lot of virtual machines for my work, so it is handy to have when launching a lot of them etc. You also get the benefit of having more memory slots available. In terms of X99 - I had i7-5820K cpu which I was maxing out all the time and since X99 is supporting XEONS as well, i decided to keep it for money reasons :) I could also keep non-ECC memory, so that was a plus.