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The catch is, in order to get moveslot no. 5, you can't choose what it is. It's determined by species, and for the most part is going to be that mon's most iconic move. All Parasect have Spore in slot 5. All Wobbufett have Destiny Bond. All Castform have Weather Ball, all Spinda have Teeter Dance etc.
But particularly powerful mons like Garchomp would be designed deliberately to have situational, borderline useless 5th moves. This makes them not have coverage on everything and gives shittmons a slight upper hand.