I’m curious why people guess words that don’t use all the letters they know are in the answer?
Because it often helps narrow the remaining possibilities down more quickly.
Assume the word is EATER and your first guess is WATER. You've hit 4 greens, but you're still left with:
CATER
DATER
EATER
HATER
LATER
RATER
+ many slightly more obscure others
If you try guessing one by one, you could get it in 2 or you might get unlucky and fail.
If you spot this, and your second guess was CHILD, you've now ruled out 4 options in one go (C,D,H,L). And (in the case where the actual answer was something other than EATER), then if either of those letters C,D,H,L isn't red, then you've now got the answer.
It's about trying to anticipate what the remaining combinations could be and then reduce those down as quickly as possible.