The answer is very, very, very simple: it was a way to standardize rules nationally for a popular class.
Improved Touring was an idea that sprang out of a desire in many circles to create a category of prep slightly above Showroom Stock (itself only about 10 years old at the time) where old SS cars (who had a rules-limited finite life) could go to play. The original rules required full interiors, didn't allow welded-in cages, and in some limited cases even required street tires. It was popular up in the Pacific Northwest (called RS Touring, wasn't it?) and soon gained popularity in several different areas of the country (I raced in IT in Texas in the mid- to late-80s...)
This created a twofold problem. First, rules were not consistent across the country, so someone could not take their RS Pinto and run it in IT in Texas, for example. Second, SCCA Colorado (HQ was in Englewood at the time) did not want to add Improved Touring to the GCR to nationally-standardize the rules because all GCR-printed category specs made the car eligible for the Runoffs; Englewood didn't want to add another category to the year-end SCCA Runoffs.
So, the infamous "Regional Only" status was developed. Improved Touring was the first class to gain "Regional Only" status in the General Competition rules, which allowed Topeka to print a nationally-standardized set of rules for everyone to follow, yet not worry about having to add the category to the Runoffs. In fact, if I recall correctly it wasn't until the publication of Spec Miata rules that any other class had the "distinction" of a nationally-standardized set of Regional-only rules in the GCR...
So there you have it: it was just a way to standardize a ruleset nationally. Of course, being so unusual, and exacerbated by the SMs gaining National status so quickly after gaining Regional-Only status, this whole situation has gained a life of its own over its roughly quarter-century existence...