You will find different times in the Divine Service where we stand up, sit down, or kneel. There are reasons for this, besides getting a workout. :)
Generally, we stand for prayer and praise. We sit for instruction. And we kneel when we confess our sins, during the consecration, and when we receive the body and blood of Christ.
Standing is also done to give honor. We honor the Name of the Trinity by standing for the last stanza of a hymn that has a triangle before it. The triangle reminds us to stand in honor of the Blessed Trinity, who Name is sung in that stanza. We also stand in honor of the Gospel, when it is read.
We sit for instruction/teaching and long hymns.
Kneeling is a sign of humility. We kneel to God. We come to Him humble, on our knees, to receive His grace, mercy, and peace.