A sentence carries more than it says out loud.
Every claim rests on assumptions its author never stated. Some are trivial, others carry the actual weight of the argument. Read the sentence "you are finally starting to behave reasonably" and you have accepted, without noticing, that until now you did not.
Propositional analysis takes a text apart into individual claims and makes these hidden layers explicit. What is asserted, what is presupposed, what follows. Once the assumptions are written out, they can be examined one by one: agreed with, rejected, or shown to contradict each other.
This is the difference between feeling that something is wrong with an argument and being able to point at the place where it breaks.