How is using the word wither more impactful than using other works like weak

First of all, wither is better compared to weaken, rather than weak. But to answer your question, weak is a very general word that can describe many different circumstances. Using wither is more impactful because it gives a sense of what specifically is happening without even expanding on it. The more specific and precise you are with verbs, in most cases the more impactful they are.