Spandrel beams are used in multi-story buildings at each floor level. These are attached to the outer perimeter of floor slabs as a belt to support the floor beams. It also strengthens the connection between the slab and outer columns. On the roofs, the parapets are placed above these beams.

What is the use of a spandrel?

In a building with more than one floor, the term spandrel is also used to indicate the space between the top of the window in one story and the sill of the window in the story above.

What is the difference between pier and spandrel?

In simplified terms: pier = column; spandrel = beam. I'd call the bit above the opening a spandrel.