For every ability dedicated solely to protecting a mount, that’s one less ability for general use. One of the main critiques of the Cavalier archetype in a game that doesn’t always allow use of a mount. Abilities that are generally useful but that synergise with mounted combat solve this issue...
Plus, since the 5e Cavalier is not rendered ineffective when unmounted, you can kind of just treat horses as valuable but replaceable equipment. Even a warhorse is not going to be as expensive as the armour you’re hopefully putting it in, which you can heartlessly (or tearfully) strip off the...
That would be because “Defender” is a very useful niche for general fighter stuff while simultaneously making mounted combat much more viable. Solving one of the main problems with the archetype.
The ability to make it harder for foes to hit your allies whilst punishing them for attempting it...