Just Richard Mellish said "Art songs and pop songs are written with chords, but folk songs fundamentally have only melodies" which may be true for folk music from certain places,
but there are lots of different musical traditions that are feature a repeating rhythmic chord pattern and feature a soloist or vocalist improvising melody and lyrics above that. Blues is an example, as are island forms like bomba and plena, salsa, montuno, reggae and it's predecessors, soca, calypso, as well as Latin forms like samba.

There are, of course, a lot of different chord patterns, not just I-IV, the La Bamba progression being probably the best known--An interesting thing to make note of is that, because the progressions repeat, they actually have no separate ending, or resolution, and no beginning, or introduction, so they allow the performer a lot of latitude in phrasing.

Another thing is that, though I-IV is pretty straightforward, the I7-IV7 can actually be considered II7-V7 instead. That is to say, you'd be playing a circle of fourths D7-G7 that implies the key of C, and never actually resolving back to the C.