I WAS WONDERING THE EXACT SAME THING! Then I found out
Basically when you set it to Asus Optimal mode from normal mode and SAVE the changes then it unlocks your CPU and RAM frequency so that when you are in your OS (windows) another program can change your RAM and CPU frequencies. For example the software Asus AI suit can adjust your CPU and OC it when required.
You can see the changes for yourself under the AI tweaker in your BIOS advanced options after you change and save them.
From Normal --> Asus Optimal
Target CPU turbo mode speed: 3700MHz -->4200MHz on my I5 2500k
Target DRAM speed _ 1300MHz -->1600MHz on my 1600MHz corsair RAM
It will boost your performance by a little bit and allow you to OC through the OS or let a program manage it for you.
Persason