1. they can be useful at locking down the board
2. knowing all of the words is necessary to be able to challenge when somebody attempt to play a phony, which, as a native Spanish speaker, you might be even more inclined to try against a non-native speaker.
Everyone knows words like "fork", "vogue" and "alligator" but only scrabble players know whether or not "forkier", "defork", "voguier", "voguiest", "alligatored" and "alligate" are real words.
The CSW is the official English word list used internationally (outside of the US/Thailand/Canada because we just HAD to be different) and contains over a quarter of a million words. Unless you're at the UPPER OF UPPER ECHELONS, there's a chance a crafty player could slip a phony in by hooking an "S" on the board or some other subtle stem - especially with the pressure of time controls.