We don’t always. Some are prayers we learn as children; some are written by the saints. The prayers referred to in the article are part of the Liturgy of the Hours: prayers said, usually by religious, at certain times of the day, so that at all times, 24/7, prayers are going up to God. These are mostly Scriptural, with intention pertaining to the Liturgical season. It’s sort of the earthly version of Revelation, where the living creatures did not cease to play to God before His Throne.
Pray. Sorry, tired eyes!