In baseball, once you hit the ball, you have an opportunity to run around the bases to score. This WOULD give the pitcher an incentive to throw bad pitches that you can't hit, but if the pitcher throws too many pitches outside a certain zone (that the batter doesn't swing at), the batter gets to take a base for free. So the best method for the pitcher to end the other team's chance at bat (chance to score) is to either to throw balls that get the batter to swing an miss or to throw balls that the batter doesn't swing at that end up in that zone.

HOWEVER, there are certain instances (like this one) in which the safest thing for the pitcher to do is throw terrible throws on purpose and let that batter take the base. For instance, there might be a man on second base and a man on third base and really good hitter up at bat. The pitcher could try his luck trying to strike-out (end that batter's chance at hitting without letting him run the bases) the really good hitter, but this risks the batter gets a home run which allows every man on base to score including the batter. OR he could allow the batter to walk to first base which gives the opposing team's next hitter a chance to score more points, but perhaps he's a much worse hitter, meaning the pitcher could strike him out without the other team scoring at all. The latter option is called an intentional walk.

WHAT HAPPENED HERE: The pitcher tried to intentionally walk this batter, but the batter swung for it anyway, hit it, and actually hit it pretty well. Being combined with the fact that the entire other team probably wasn't ready for this, the man that was already on second (at least, the angles don't show any other runners on base) was allowed to run all the way to home plate and score and the batter got safely to first base.

tl;dr: pitcher tried to mitigate scoring threat, didn't work.