1.It is the pillar upon which Islam is built. The Prophet said "Islaam is built upon five : the tawheed of Allah…..(Related by Muslim no. 19)
2. The greatest need for mankind, since there is no life for the hearts nor delight nor tranquility except through knowing their Lord , the one to be worshipped, the creator.
3. It is the most noble branch of knowledge. As Imaam Abu Hanifa mentioned, when he called his book which he compiled concerning the Usool ud Deen (fundamentals of the religion) " fiqh al akbar" (the greatest fiqh)
4. It was the key and essence of the call of the messengers "Verily we have sent to every nation a messenger saying worship Allah and avoid the worship of false gods" (16:36)
5. because Allah has created us to worship Him, " I have not created the jinn and mankind but to worship me" then how can we worship Allah, if we don't know who He is? It is only through knowledge of His names and attributes we can learn about Allah.
6. Firmness upon the deen, excellence in this world and salvation in the Hereafter is built upon this. a) because when you know Allah, His names, attributes and Actions this necessitates loving Him, fearing Him, placing reliance upon Him etc. b) when you know what allah loves and is pleased with or angered and displeased with, you will rush to fulfil what He loves and avoid what He hates.
7. Its great ephasis in the Qur aan. The qur aan begins with tawheed (al fatihah) and ends with Tawheed (al ikhlas) likewise the greatest ayat in the Qur aan describes tawheed (of knowledge)
8. The prophet began his day with tawheed by reciting surah al kafiroon and ikhlas in the sunan of fajr and end with reciting these two surahs in the witr prayer. (related by muslim)
9. The prophet mentioned it during his last illness ( shows its importance) when he said " the curse of Allah be upon the Jews and the Christians for they took the graves of their prophets as places of worship." (bukhari)
10. It's the first thing the prophet invited to and preached to for thirteen years. "say, none has the right to be worshipped and thus be successful." (ahmad 4/63)
11. And he ordered his compnions to preach Tawheed when sending them to various communities. He sent Muadh ibn jabal to Yemen saying " indeed you are going to the people of the book so let the first thing that you invite them to be the tawheed of Allah" (Bukhari 1/13)
12. You have to compromise the three aspects together and not just believe in one, merely with the meaning "the one who has the power to create and originate" as the arab mushriks used to agree that Allah alone is the creator of everything, despite this they were still mushriks. Plus this was not the true reality which Allah sent the messengers with. But rather we must believe in his Ruboobiyah, uloohiyah Asma was Sifaat together.
13. When you belive in the three aspects of tawheed. i.e Ruboobiyah- youwill negate any source of harm, blessing, power from any other than Allah. Uloohiyah- this will stopyou from committing shirk. Asma was sifaat - to have the correct knowledge of this will prevent you from distorting any of Allah's Names and Attributes, or explaining them, or denying them or asking how they are.
14. If a person dies upon this he will enter Paradise. When the Jewish boy accepted Islam, the Prophet (sallallaahu alayhi wasallam) said, "All praise is for Allah who saved this boy from the fire." Although this person had no good deeds. (Bukhari)
15.Tawheed is a condition for an action to be accepted, because for a deed to be accepted it requires 2 conditions. a) The intention is purely for Allah (tawheed al ibaadah) b) It must be in accordance with the shariah of Muhammad
16. It strengthens ones heart and makes one firmer and well rooted. Then as a result ones deeds will multiply and likewise his worship of Allah will be greater.
17. It makes it easier to fulfil the commands of Allah and leave the prohibitions as Aaisha said: "had the first revelation been do not drink and fornicate or commit adultery etc they would have said we shall not give up adultery and fornication etc." (Bukhari from Yusuf ibn Maalik)
For it was only after Tawheed was understood by the companions and had settled in their hearts that Allah began to order them with various commands.
Source: https://www.salafipublications.com/sps/sp.cfm?subsecID=TAW01&articleID=TAW010006&articlePages=1