So far it sounds for me like it was only to make logistics and sieging cities even more frustrating
I usually look at these things as crutches for the A.I. honestly. Take civ5 introducing the city health and bombard mechanic. Obviously this is great for the player, but I really feel it was introduced to make early rushes against the A.I. at least slightly harder.
Similar case with civ6 promotions. In civ5, for whatever reason, the A.I. seemed bias toward the heal option. Now a simpler solution would be to code the A.I. to be biased toward promoting it's units, but you also have the issue of A.I. units tending to find itself in tricky spots, so in civ5 that would probably just amount to more dead units. Further, both healing and promotions are more powerful in a players hands (as is everything).
So I really look at the change to promotions as a change that is beneficial to the A.I, It allows them to heal, as well as upgrade themselves. Further, the consumption of the units movement points halts player advances and requires some thought, as I've noticed quite a few units die due to promoting in the wrong spot. With the A.I. demonstrating the ability to retreat when wounded as well, I feel like the longevity of A.I. units has been increased dramatically with such a simple change.
What does all of this have to do with the movement mechanic? Well, as evidenced of the fact that in almost every video there is a case where a player attempts to finish off a nearly dead unit but is unable to catch it - The system, in my mind, benefits the A.I. Since it's been claimed (and slightly demonstrated, but not perfectly) that they will retreat when wounded, and not wander around a battlefield or city waiting to get killed off - making it harder to catch them is a good thing.
Even when two units are right next to each other on flatland, if the retreating unit moves into a hill or forest - the pursuing unit would at that point lose the ability to catch up. I think this is awesome. Also, if it contributes to tougher sieging, I think that helps the A.I. too
Obviously every rule and mechanic will ultimately end up stronger in the hands of the players - but I think mechanics that bridge that gap by essentially making things slightly more difficult for the player are a good thing. Especially when it comes to war. The bonuses the A.I. receives for high lvls of difficulty helps them in all aspects of the game except war (apart from unit capacity). So in addition to just making the A.I. better, if there are mechanics that can be designed that favor the A.I defensively, then I think they should be put in place.