Default (JEDEC): DDR4-2400 CL17-17-17 @1.2V means it will run at that speed and at that CAS latency,without you touching anything.
By turning on the XMP profile,you will get either XMP Profile #1: DDR4-3200 CL16-20-20 @1.35V or XMP Profile #2: DDR4-3000 CL16-19-19 @1.35V and i THINK that is depended on the motherboard.
They probably said that its better to buy 2666mhz since its probably cheaper,and if you went with 3200mhz,it will be more expensive without having the ability to run at advertised speed,which is 3200mhz on this motherboard.