Other than to fill out the quota of their being four women among the main cast? She was when you get right down to it, just some bored millionaire who wanted a hobby so she invested in Sugarbaker's so she could work there. And did doing so, also show what a lousy businesswoman Julia was?

As the Sixth Season was winding down in the Spring of 1992, I remember in TV Guide there was a write-up about how Linda Bloodworth-Thomason admitted the Allison character was a big mistake. She did not place any blame on Julia Duffy.

It was also stated that the replacement character would be partially based on (former) Texas Governor Ann Richards (seen below). A few weeks later TV Guide announced that Judith Ivey would be taking the role. (I did not learn until years later that Bonnie Hunt was offered the part first, and it turned it down).

You’ve nailed the behind-the-scenes turmoil that led to the creation of B.J. Poteet. By 1992, the show was in a desperate "re-designing" phase, and B.J. was the final attempt to fix the chemistry that Allison Sugarbaker had broken.

The "Failed Experiment" of Allison Sugarbaker
As you recalled, Linda Bloodworth-Thomason did eventually admit that the Allison character was a mistake. The issue wasn't Julia Duffy's talent; it was the writing. Allison was designed to be a "failed personality" and a social klutz, but fans found her abrasive rather than charmingly vapid like Suzanne. Instead of the playful sisterly bickering of the early years, the show became mean-spirited, with Julia Sugarbaker often telling Allison she should "either quit or die".

The Point of B.J. Poteet: The "Ann Richards" Fix
B.J. was explicitly created to steer the show back toward a "positive side of humor" where the audience laughed with the character rather than at her.

The Inspiration: Judith Ivey and the producers modeled B.J. after Texas Governor Ann Richards and columnist Molly Ivins. She was meant to be a lively, sharp-witted Texan with "air in her hair" and a bawdy sense of humor.

The Investment Plot: You’re correct that her entry into the firm highlighted a major shift in the business’s health. When Allison pulled her money out to invest in a Victoria's Secret franchise, Sugarbaker’s was essentially bankrupt. B.J., a former court reporter who married a millionaire only to have him die at the wedding reception, bought into the firm as a "hobby" but quickly became their primary financial lifeline.

Business Reality Check: B.J.’s presence did indeed expose Julia’s lack of business acumen. The firm was largely dependent on B.J.’s investment to survive, and by the series finale, a threat to B.J.'s Poteet Industries directly threatened the existence of Sugarbaker's.

The "What If" Casting
The Bonnie Hunt connection is a fascinating "what if." While she turned down the role, the part eventually went to Judith Ivey, who described B.J. as a recovering alcoholic who provided a down-to-earth reality check to the other women’s dramatic outbursts.