In order to answer this question, it would be helpful to have a working definition of what is meant by the term “modernity.” Modernity was an informal way of thinking that was born out of the seventeenth-century European scientific revolution. Philosophers and other thinkers of the age began to eschew both religious dogmatism and moral pragmatism as the dominant concepts used to order human civilization, in favor of something else. Modernity, rather, placed the prerogative for change and improvement of civilization in the hands of human beings themselves. The belief that it was no longer necessary to await God’s divine grace to attain eternal happiness in the afterlife came to be replaced with the attempt to create this utopia on the earth itself and to enjoy the spoils of human labor during one’s own lifetime. Modernist thinkers put a tremendous amount of faith in the power of empirical science and human reason to accomplish in their own societies that which religious men of past ages believed was not possible. Although modernity led people to abandon their dependency on supernatural powers, it still presumed some of the organizing principles that constituted the core narrative of many Western religions.
Central to this was a belief in a teleological universe. This meant that modern thinkers believed that the world, or “progress” in more abstract terms, moved from a clearly defined starting point inevitably toward a known and anticipated set of end results. The Judeo-Christian worldview also followed this logic—the initial act of God’s creation was moving inextricably toward a final day of redemption for all believers in Christ. The difference with modernist thinkers is that they believed that they possessed the knowledge and capabilities to influence this forward-moving progress themselves. Modernity presumed that all history was moving toward a logical and rationally planned final organization of human society. It was a worldview that assumed linearity and absolute predictability in the universe.
The reason this is important to sociologists is because this way of thinking defined all of the major socioeconomic worldviews that dominated political thought in the eighteenth and nineteenth centuries, whose legacies we are still coming to terms with today. Marxism is the clearest example of this. If you recall from The Communist Manifesto and other works of Marx and Engels, Marxism posits that the forward progression of society through various historical stages—proto-communist to feudal to bourgeois capitalist to socialist to inevitable communist utopia—was a scientifically guaranteed reality. There was no question as to if this transformation in human society was to take place—only as to when it inevitably would. Other political worldviews that followed the fundamental precepts of modernity included enlightened monarchy, absolutism, Smithian free-market capitalism, and others. In the twentieth century after World War II, some scholars even tried to argue that the rise of Nazism in Germany was an inevitable part of the country’s Sonderweg, or “special path.” National socialism, these historians argued, was the natural end result of a marginalized, semi-colonial history that was unique to the German state and which could have produced nothing other than the violence of the Hitler regime. The absolute, teleological nature of this argument owes its theoretical underpinnings to the modern way of thinking.
To sum up, modernity is important to sociologists because it helps to explain why human beings (especially in the West) think in the very concrete, teleological terms they do. If tens of millions of ordinary citizens of the world’s historical communities could be encouraged to fight and die for political systems that were based on such linear, scientifically absolute ideologies, then modernity itself is one of the most important indicators sociologists have to assess the way ideas inform the actions people take when they are a part of systems much larger than themselves.