What is the use to know that a compound optically active or not?
Yes, it is important.

For reasonably enantiopure chiral molecules, it is expected to record and publish a specific rotation when it is first described. For the person recording these, they are a little bit annoying, especially if you have a polarimeter as old as the one I had to work with during my PhD. Therefore, you want to cut time as much as possible. By immediately determining that a compound is achiral, you eliminate the need to record a specific rotation because theory predicts it is zero.

Conversely, if you were to simply record all specific rotations of your compounds and then just remove the ones that end up as zero you would likely miss the chiral molecules you synthesised as a racemate. If, however, you take a look at the molecule on paper, are able to determine it should be chiral—given the methods and chemicals employed—but still find a specific rotation of zero you have something that must be discussed in the final paper or thesis.