It is a sign of respect, just as one stands for a monarch or a judge. It is traditional for the preacher to read the gospel reading immediately before the sermon. We have lay readers, scheduled in advance, who read the Old Testament and Epistle lessons. I lead the congregation in reading the psalm responsively. The congregation remains seated as I read the gospel lesson, which comes right before the sermon.
Many churches use corporate responses in worship.
For example, the minister might say, The Lord be with you and the congregation responds in unison, And also with you. Or, after a scripture reading, the reader might say, the Word of the Lord and the congregation responds in unison, thanks be to God. Many congregations also have a responsive reading, usually a psalm, where the reader and the congregation alternate. The purpose of the responses is to make sure that the congregation actually worships and that it does not become an audience, as in medieval times, that just watches worship. They also are intended to make the congregation realize that they are the Body of Christ, not just a random collection of individuals.