Is it not more appropriate to ask: What is the wisdom behind prohibiting a woman's travel without a mahram, and what exactly does his presence provide?

Your question is incomplete. The answer to the question: "Why is it not permissible for a woman to travel without a mahram (definition)?" is that this is a ruling of the Shari'ah, and a Muslim must submit to it, in accordance with the Almighty's saying: {The only statement of the believers when they are called to Allah and His Messenger to judge between them is that they say, "We hear and we obey."}

As for the wisdom behind it, it includes protecting and safeguarding the woman, which is for her benefit, not to restrict her. In this era, the need for a mahram during travel increases due to the spread of immorality and crime. Even on an airplane, a woman may face risks of delays or flight diversions, or sitting next to a bad person. So, the wisdom of the Shari'ah in prohibiting a woman from traveling [alone] is the correct wisdom that ensures safety.

And Allah knows best.