ACPI is required for power management to reduce electricity usage and wear-and-tear on the system components. Your motherboard supports the 2.0 revision of the ACPI standard which of course has some enhancements over the original spec, for supported hardware (that board is, but your other devices like CPU, monitor and hard-drive may or may not be—but probably are). The major changes were the addition of 64-bit addressing and device state and multiprocessor support.
So your options are to have power-management or not, and since you can always simply not use it (turn off the options in the power control panel applet), you may as well enable it in the BIOS.
The only reason to disable it is if it is a buggy implementation which ends up causing problems, but neither the motherboard’s manual nor its forums mention any ACPI related issues. The FAQs for it do include one question about ACPI, but that is a specific Vista related issue which is not even an actual problem, just a notice. Therefore, defaulting to disabled is just a curiosity which if really wanted, can be asked.
If you want to learn specifics about the ACPI spec, you can see the changes in Intel’s Technical Update, or a short, general overview at the ACPI site. For real technical details, you can peruse the full specifications.
ACPI is up to version 4.0 at the moment, but 1.0 performs most of the required functions for most purposes. The useful enhancements of later revisions are support for 64-bit, SATA, and PCI-E, but they also include a lot of stuff that most users probably don’t (currently) have or need like MPS, server, ambient-light sensors, user-detection devices, and USB3.