Advantage of special instructions

I ever wondered the need for special instructions added by Intel into its already bulky CPU instruction set, for example, the AES instructions. Recently, I enable the FileVault2 fracture of my 2011 15' MBP, before that, of course, I do some research about the performance impact and find this article: FileVault 2 Benchmarks Show Full Disk Encryption is Faster Than Ever in OS X Lion. Pay attention to the difference of performance impact on MacBook of 2010(core duo, without AES Instruction set) and 2011(i7, with AES instruction set), you can see that AES instructions help out here.
One real world example is always stronger than the assertion in textbook.