What is the reason behind the opening Khutba of the Prophet (Sallallahu Alaihi wa Sallam) when he begins with Thanaa and Tahmeed and the three verses of Qur'an regarding Taqwa and the mention of Bidah?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is narrated by Ahmad , Abu Dawood , al-Tirmizi and Ibn Majah , that Abu Obeda reported that the Prophet (Sallallahu Alaihi wa Sallam) used to say in the Khutba (sermon) of request (when badly in need of something to be achieved):
"All praises are due to Allah, we praise Him, we seek His aid and we ask Him for forgiveness, we take refuge unto Allah against the evils in ourselves and against the badness of our deeds. Those whom Allah has guided no other creature will lead them astray and those whom Allah has misguided no one will guide. I testify that there is no God except Allah and I bear witness that Muhammad is the slave and Messenger of Allah. Then he read three verses from the Holy Qur'an as follows:
{O you who believe! Fear Allah (by doing all that He has ordered and by abstaining from all that He has forbidden) as He should be feared. [Obey Him, be thankful to Him, and remember Him always], …} .
{O mankind! Be dutiful to your Lord, Who created you from a single person (Adam), …} .
{O you who believe! Keep your duty to Allah and fear Him, and speak (always) the truth.} .
After that he requested from Allah the thing which he needed badly".
The Muslim is obliged to obey any rule and believe in what is he told in the Qur'an and Sunnah of the Prophet (Sallallahu Alaihi wa Sallam) whether he knew the rationale behind that matter or not. However, when the Muslim is aware of the reasons for a certain rule, he will become more convinced and obedient. In this context, we cannot clearly state the reasons for opening the Khutba of request with these words or verses. The text is self-explanatory. The Thanaa and Tahmeed in the beginning of the Khutba are to remind us of the so many bounties conferred on us by Allah that we have to glorify Him because he is worthy of gratitude and we are badly in need of His aid. The sermonizer or Khateeb requests Allah to help him so as to be successful in his sermon and in all aspects of life and wishes the same to the audience. He is also requesting Allah for forgiveness and repentance from his sin while he is preaching. He also seeks refuge from Allah from the evils of his ego. The ego may lure the human being to be over confident, arrogant and become self-admiring. Taking refuge unto Allah from the evils of the ego has the same effect.
The sermonizer or the Khateeb by saying: "Those whom Allah has guided no one can misguide" means that this Khutba or sermon will not be guidance to any one unless Allah so ordained. Allah Says (interpretation of meaning): {Verily you (O Muhammad (Sallallahu Alaihi wa Sallam) guide not whom you like, but Allah guides whom He wills. And He knows best those who are the guided.} [28:56].
If it is ordained by Allah that somebody will be misguided this Khutba or sermon will not guide him. As for the testimony that there is none worthy of worship except Allah, and that Muhammad is His servant and Messenger, it is an acknowledgement that there is no Deity except Allah, and it is an acknowledgement that the Messenger has honestly accomplished his task of Prophet hood.
Reciting the verses which urge the Muslims to be pious is made because piety is the ultimate goal of obedience to the divine commandments, and to comply with the prohibitions. It is as if the Khateeb (sermonizer) is saying that my sermon is emphasizing this point. Warning against bida (invocations) is important, so as to be careful otherwise the religion of Islam will be corrupted.
Allah knows best.