The hymn is virtually the only part of the LOTH that is not from scripture. Originally the hymn was a brief theology lesson intended to combat heresy.
That’s still the case if you use the traditional Latin hymns. I do, using the Liber Hymnarius. Note that I use the current 4-week LOTH and sometimes a 1-week monastic version of the LOTH. The Liber Hymnarius is for the current LOTH and monastic LOTH.
Most modern hymns tend to be set to the character of the season or hour. A few are more or less translations of their Latin equivalents. It’s a bit more of a mish-mash now, some vernacular hymns (including many in French) are rather mediocre. In the French LOTH, the numbers refer to hymns in a separate hymnal that exists. There are one or two French hymns that I like, but for the most part I prefer Latin (I am not very familiar with the LOTH in English as I live in a French-speaking part of Canada).
One doesn’t need to sing/chant the hymn One can treat the hymn as a form of poetry. Some old Latin hymns are indeed beautiful poetry even without a melody.