And the public as well. If I hire a lawyer, I have a signal that they probably have (at least) a rudimentary understanding of law, and are licensed to practice law. If I see a medical doctor, I know they have a degree and are licensed to practice medicine. If these people wind up negligent or put people in harm, I know that they have the chance of losing their license to practice law or medicine.
If I need an engineer, I should have at least some assurance that they know what they are doing, and that they aren't going to act unethically or against my best interest. I should also have some assurance that the fact they hold a license, means that they (probably) haven't been negligent, or harmful with their practice.
Professionalization is part of it, but not the whole part. The main part was to manage the number of members. Also, this professionalization greatly decreases the propensity for bridges collapsing due to negligence/malpractice but does not completely prevent it. [miami pedestrian bridge, galloping gurtie, KC hyatt walkway, genoa viaduto].
If I need an engineer, I should have at least some assurance that they know what they are doing, and that they aren't going to act unethically or against my best interest. I should also have some assurance that the fact they hold a license, means that they (probably) haven't been negligent, or harmful with their practice.
This is what Texas used for its software licensure examination: https://nceesorg.wpenginepowered.com/wp-content/uploads/2015...
All of that seems like, the barest minimum that I don't think anyone would disagree with to call someone a software engineer.