Because Allah says in the Quran meaning O you who believe fulfill your promises. This is the first verse of Surah Almaidah. For a muslim, a promise has to be fulfilled. Do not give your word that you will do something unless you have as good as already done it. When the customer expects you to do something in two days and you build it in one, you have under-promised and overdeliver.

You have exceeded their expectations. Now they will be happy to refer you more customers.