Shaykh Saalih Aal al-Shaykh (hafidhahullaah) says in his explanation of al-Aqidah al-Hamawiyyah (2nd cassette, Side A):

The intent behind "important knowledge" is that you know how deviation entered into the Muslims, how did it enter? You will not understand the sayings (of the Salaf) in aqidah and nor the detailed elaborations until you know how that evolved over the years. In the first century was such and such and in the second century was such and such, and the graduation in that. You will understand the origin of every saying, why did differing occur with respect to it? And the conflicting views, what was their reason? But as for when you take the sayings of the scholars, or the sayings of the opposers without being acquainted with the successive development of the saying (in that matter) and how it originated, then you will (only have) a weak comprehension of what is intended in their sayings (of the opposers) and refutation of them and so on. So from the important knowledge is that you know how deviation entered into the Muslims in diverse areas, in creed, in action, in detail and so on ...

You can refer to a series of articles on the history of the sects in the early history of Islaam here:

• The Splitting of the Muslim Ummah: Part 1 - The Completion and Perfection of the Religion and The Way of the Companions - (see here)
• The Splitting of the Muslim Ummah: Part 2 - The Emergence of the Sects and the Heads of Misguidance - (see here)
• The Splitting of the Muslim Ummah: Part 3 - An Overview of the Various Deviant Sects and Corrupt Creeds and Methodologies - (see here)
• The Splitting of the Muslim Ummah: Part 4 - The Preservation and Safeguarding of the Islamic aqidah - (see here)

Whilst the articles above provide a broad history, what is intended for the purposes of this article is the origin, Evolution and graduation of each saying, so for example: How did the saying of the Khawaari, that major sin expels from Islaam, arise? Or how did the saying that Allaah is not above His Throne arise? Or how did the saying that actions are not from Eeemaan arise? Or how did the saying that the Qur'an is created arise? (study this article). So what were the historical backgrounds to these sayings and what did the Salaf say in response and how did these various sayings evolve and develop? These are the issues that a person of the Sunnah must know and grasp so that his feet are planted firmly. Likewise in a contemporary setting, how and why did the saying of Hassan al-Bannaa arise, his so called "Golden Principle" of cooperation, and how and why did takfir of the whole Ummah arise, and how and why and for what purpose and end did the concept of the "Islamic Jamaa'ah" arise in the 20th century, and why did some Ikhwanis try to distinguish between al-Firqah al-Naajiyah and al-Taa'ifah al-Mansurah, and why did the same Ikhwanis invent the principle of al-Muwaazanah and so on.