TL;DR: It will run at that speed, and it will be an asset in some workloads.
Memory overclocking and XMP
Intel specifies that their CPUs (here, the 7700K) are guaranteed to work with DDR4-2133/2400, DDR3L-1333/1600 @ 1.35V. Anything beyond that isn't officially part of their specifications, or guaranteed to work. However, in practice, it will.
What's going on here is overclocking–basically, pushing stuff beyond it's manufacturer rated speeds. This practice isn't unsafe, and, in the case of memory, is basically supported by the manufacturer. Here, the RAM manufacturer certifies that the RAM will work at 2666 MHz. Though the motherboard manufacturer only guarantees that RAM at 2133/2400 MHz will work, it supports overclocking up to 3733 MHz and beyond (and effectively guarantees it will work.) Similarly, Intel allows their CPUs to work with overclocked RAM (and effectively guarantees it will work.)
Because memory overclocking takes work, Intel created something called XMP, or Extreme Memory Profile. Basically, this is a list of settings stored on the memory modules that tell the motherboard what speeds it says it can overclock to and what timings it needs to do so. Most motherboards will then automatically discover and load those settings, though some may require manual configuration. The end result is the same though: the memory runs at its "overclocked" speed.
RAM speed as an asset
Though I won't dive into it too much, how much–if at all–RAM speed matters depends on your workload. This video does a much better job examining it then I could ever do, but the basic conclusion is that it matters a little in some games (more on Ryzen then other platforms) and varies widely for other workloads. If you're building for gaming, don't sweat it too much; if you're building for some other task, Google around and see what other people have experienced.