Nolan Gorman provides left-handed power that the Cardinals have been sorely lacking
Although the Cardinals' lineup has been one of the better lineups in baseball the last two years, the two things people could point two as major flaws were the last of a third dependable bat and the lack of a left-handed power threat.
This has never really hurt them in the regular season, but when you get to October, matchups are everything, and not having someone you fear from the left side of the plate limits the amount of pressure you can put on an opposing staff.
With Gorman, the Cardinals may finally have their answer for the elite pitching they'll face this October. Gorman can eventually bat fifth in the Cardinals order as protection for Goldschmidt and Arenado, or they could get creative and slot him in third, to break up the run of right-handed hitters and make it extremely hard for clubs to bring in a lefty to face Gorman or righties to face Goldschmidt and Arenado.
Gorman will be 23 soon, so that may feel like a lot of pressure to put on a second-year player, but that is the benefit of Gorman being the third bat - the two guys he's behind in the pecking order are MVP candidates. Plus, he's already shown a clutch gene two times during the young season.
Not only that, but the Cardinals also have a deep lineup of high-upside bats to take pressure off of Gorman, which brings me to my third point.