Before the rulebook
Early mixed martial arts events varied wildly from show to show, with different bans, different surfaces and different ideas about what counted as a stoppage. That variety made the sport hard to regulate and harder to compare across cards, since a foul in one promotion could be a legal technique in another. Athletic commissions were reluctant to sanction something with no fixed definition, which kept the sport at the margins of legitimate combat sports regulation for years.
What the unified code fixed
The unified rules gave commissions a single document to license against: banned strikes to the back of the head and spine, closed-fist strikes to a downed opponent's head in some jurisdictions, and clear definitions of grounded status. None of this removed danger from the sport, but it removed ambiguity, and ambiguity is what regulators cannot work with. Once the rulebook was common, judges, referees and commissions could be trained against the same standard everywhere.
The knock-on effects
A common rulebook is also what allowed weight classes, round structures and scoring criteria to be treated as settled infrastructure rather than promotional choices. Fighters could move between organisations without relearning what was legal. Commentators and judges could apply one vocabulary. The unified rules are rarely the most exciting part of the sport to discuss, but they are the reason there is a sport to discuss at all in a consistent way.