بسم الله الرحمن الرحيم
The opening supplication is a sunnah and is not obligatory whether that is in the five compulsory prayers or the supererogatory. And that which is befitting is that the people use all the (forms) of the opening supplication reported from the Prophet -(May Allah’s Peace be Upon Him)- so that by way of that he attains the reward of performing the sunnah with all its different forms. And if he only knew one form from the sunnah and he restricts himself to that, then there is no sin upon him. Because that which is apparent is that the Messenger -sallaallaahu alaihi wa sallam- used to vary the forms of the opening supplications and the tashahud for the purpose of ease for the slaves (of Allah). Likewise the Messenger used to vary the dhikr after the prayer for two benefits:
The first benefit: So that an individual does not continue upon one type; for if a person continued upon one type, it would become a habit to him and because of that he would become negligent and find himself saying this dhikr without intent, due to it becoming an action which he does out of habit. So when the adhkaar are of various types and a person uses them intermittently, his heart will be present whilst reciting and this is conducive for understanding and contemplating on what he says.
The second benefit: Is ease upon the ummah, because a person can use one form or the other in accordance to what is befitting in his personal circumstances.
So due to these two benefits some acts of worship have come with various forms like the opening supplications, the tashahud and the adhkaar after the prayer.
Paraphrased from Shaykh Al-Uthaymeen’s Majmoo’ al-fataawa war rasaa’il volume 13 Page 74