That is a good question. If you want more info on boot.ini switches, you can go here
The short answer, to help avoid malicious code execution...
This option is only available on 32-bit versions of Windows when running on AMD64 processors and only when PAE (see the /PAE switch) is also enabled. It enables no-execute protection, which results in the Memory Manager marking pages containing data as no-execute so that they cannot be executed as code. This can be useful for preventing malicious code from exploiting buffer overflow bugs with unexpected program input in order to execute arbitrary code. No-execute protection is always enabled on 64-bit versions of Windows on AMD64 processors.
I believe this switch also works in Intel processors because the computer that I used to copy the boot.ini file is running a Intel Mobile P4 2.8Ghz processor.