And nobody said otherwise -- for the core argument.
But in real life we don't usually know whether the thing (let's call it A) itself that an argument tries to prove is true.
We only know that the argument in favor of A is formulated correctly (follows its premises correctly, etc).
Regarding A though, there might be other premises we haven't checked (and weren't mentioned in the argument), other facts that invalidate it, etc.
Thus it is enlightening to know if an argument for A is being made hypocritically, to evaluate the possibility of A being true in a wider context.
E.g. if I say "A is good for everybody, for X,Y,Z reasons" and I do the opposite of A, then the possibility of A really being good for everybody is somewhat lessened by the prior that "the person proposing A does not follow A".
This is because:
a) nothing guarantees that X,Y,Z (the things mentioned in the argument) are enough to prove that A is good for everybody.
b) the fact that someone proposes A but does the opposite, adds credence to the possibility that there are benefits from NOT following A.
Heck, even if the argument is that "A is good for you for X,Y,Z" and it's correct, someone that knows X,Y,Z (since he put forth the argument) but does not follow A, might know something like A+ or B that's even better, and tries to sell me short.