My understanding is that it allows for precise control over the CD drive. Windows 98 and 95 have it built in, but with the NT based OS like 2000 or XP, Microsoft didn't want to keep paying for it. I think it is only needed in "secure" mode for EAC, and as far as I know, EAC is the only program that rips in a "secure mode" type setting. Ripping in Secure without the ASPI layer will create all sorts of problems from sync errors to poor sounding mp3s. Worse yet, poor sounding mp3s are created with no error reported. So you'll be listening to your tunes before you notice it. I had ripped nearly 400 CDs in a marathon session before realizing I had to rerip them all.

I'm sure someone else can explain the "how" of ASPI much better.

What makes EAC special is the secure mode rips. Each part of the CD is examined at least twice and the results are compared before error correction kicks in. Other programs are faster (and EAC can be faster in "burst" mode) but you don't get the quality control.