A Korean software engineer wrote the following sentence.
There are several algorithms commonly used by software developers.
But his native English-speaking teacher corrected it by inserting "which are" between "algorithms" and "commonly" as following.
There are several algorithms which are commonly used by software developers.
He failed to ask the teacher why at that moment, but he, as well as I, is still wondering why. Is there any difference between using "which are" and omitting it?