Marketing / branding, mainly!
I believe the 8i was just the predecessor of the 8SE. In fact, a bit of hunting around reveals a manual for a NexStar 8i SE (Special Edition):
http://www.celestron.com/c3/images/f...022-XLT-SE.pdf
Being an older unit, an 8i (non-SE) may have slightly lower specifications than a current 8SE - e.g. coatings ("StarBright" in an 8i vs "StarBright XLT" on an 8SE), hand controller software, etc.
You can get the manuals for both models (8i / 8SE) from this site, and do a detailed comparison of the specifications:
http://www.nexstarsite.com/Manuals.htm