A lot of artists these days don’t think it’s necessary to know music theory. They write and play whatever sounds good to them. That’s all well and good, but as an arranger of music, I believe music theory now is more important than ever. With today’s technology, artist/producers may be able to make anything sound decent, but what makes music interesting and what will separate it from every other well-produced piece are changes that don’t include just going back and forth between two or three chords.
These changes can include using augmented sixth chords (these include the tonic of the key, the raised fourth, and lowered sixth, and more depending on the variations), complicated rhythms (tuplets or triplets, anyone?) or mode mixture (if a piece is in major, throwing a few chords from the parallel minor in there, or vice versa). A great example of mode mixture is “Michelle” by the Beatles. The song is in F major, but the F minor scale is included many times throughout. The bridge especially focuses on the minor, as it’s based around a D flat chord (which is not in F major but F minor).
Some artists with good ears and creativity will throw these changes in without knowing what they are, but it’s so much more helpful for arrangers and writers to have an idea of these chords and what their function is. It makes pieces much smoother and more interesting for the listener.