One reason why modernity is important to sociologists is because it, in many ways, gives them a reason for being.
One important aspect of modernity is that it holds that everything about the world (including society and all its aspects) is open to question. We are supposed to be able to think about why things are the way they are. We are supposed to be able to ask if the status quo is really ideal. If we did not believe in modernity, we would be in a society in which everything was supposed to stay as it was because it was ordained. We would not want to question because things are simply supposed to be as they are.
In this sort of a society, there would be no place for sociologists. There would be no point in studying society because society would be unchanging and unchangeable.