The video is a good indication of the momentum carried by charging horses - imagine, then, how much havoc and chaos you could create in a cohesive unit of cavalry charging through an opposing unit. To answer your question on how often the cavalry charge was used, you first have to figure out which context you're referring to, and then determine how the cavalry charge was used. For example, whilst Europeans had access to large, strong, war-bred horses such as destriers, other areas throughout history such as Japan instead had horses instead comparable in size to large ponies - thus, you can see how the context affects how the cavalry charge was used. Another question (which also ties in to your second) is how the charge was used - I'll focus on the first.

Morale. When you think of horse, you'll probably think back to films or clips of people riding horses gracefully, getting them to perform at equestrian events and so on. In a battle, a horse was anything but. I forget the exact weight, but something approximately weighing half a ton, in armour, with someone on top, also in armour, swinging a weapon at you. Its rushing at you with thundering hooves and fearsome snorts, and that's only one rider. Now imagine an entire unit composed of these horse riders, all riding at you. That's enough to make many run. A large part of cavalry charges was inflicting shock - if the charge intimidated the enemy, made them waver or retreat, or in the best circumstances, to break and run, then the charge has served one of its purposes. However, if you charge into enemy lines, and find that they are well disciplined and trained and have not broken, then prolonged melee is something you have to avoid. Whilst this applies to a lesser extent to some forms of cavalry (heavy cavalry or dragoons, for example), the same principle is that remaining in the midst of many angry men with swords on a horse is not ideal.

Ways in which cavalry could be used include harassing enemy units to force them off a position that favoured them, or to flank a unit already engaged in battle. For example, Alexander famously used a hammer-anvil tactic whereby his infantry would engage the enemy and keep them there for his Companion cavalry to attack and disintegrate the unit from the back or sides. Whilst this is a more case-specific example, the idea is that cavalry, with their high mobility (due to them being horses) allowed commanders to swiftly provide a localised superior force - he could use cavalry to augment one line's defenses, or to go around the side and harass the archers/whoever else.

Cavalry could charge, then split up and run around the unit and regroup for another charge - intimidation tactics, again. Well-disciplined infantry know to group up and hold their ground in face of a cavalry charge, rather than spread their forces thin in lines (which is why cavalry worked so well against line infantry) – however, these infantry, when regrouping and changing their formation, allow for another unit other than cavalry to come along and deal with it. You could order artillery shelling that location, or direct your archers to fire into the closely-packed mass of soldiers that was originally well spread out.

A cavalry charge tended to start off with the lead horse initiating a trot forward, which would then develop into a galloping charge before impact - this had a twofold effect of both reducing fatigue of the horses whilst also making sure that the unit retained its cohesion. The last thing you'd want is for a well-trained unit to break formation and get slaughtered one by one. However, that is not to say that there was only one formation used by cavalry - many different forms were used. A common one, and one that would make some sense, is a wedge formation in which the riders would form a triangle wedge to drive into the enemy unit - then, the combined forces of horse and steel would push away the footsoldiers.

Bear in mind also that the charge developing could also depend on the weapons used - lances, if delivered correctly, could not only kill the first person but several behind him as well, flinging them all back and causing confusion. Sabres or spears could also be used to essentially present a massive wall of spikes - in many cases too, since horses took more resources to breed and maintain, the rider would still be more skilled than soldiers on foot. By some bad luck should the rider be dismounted, he could still fight on.

However, this is never the desirable condition for battle, and so it was also often figured out beforehand by the commander whether or not the situation justified use of cavalry. Nevertheless, a head on charge into densely packed infantry would not be the ideal way to do inflict damage - again, coming back to their mobility, cavalry could instead move around the shifting lines in battle to find weak areas in the enemy's position, and exploit it to their benefit. Of course, if the enemy was routing or were defenseless (or close to), then a head on charge could just as easily be the better move.