This mochi is a special style kept on a shrine at New Year called kagami mochi (鏡餅). Typically a bitter orange is put on top to finish the presentation. The bitter orange, called daidai (橙,) is believed to promote long life because it also means "several generations."
These are storebought versions; homemade ones are two rice cakes stacked on top of each other instead of the tiered mold. The commercial versions add decoration to make up for the processing, perhaps.