Writing a song is a lot like writing a computer program. They both require clever management of loops and control flow.
The simplest sheet music reads as a straightforward top-to-bottom list of instructions. You start on measure one and read through to the end sequentially. That’s fine unless the music is very repetitive, which most popular music is. The loop is the basic compositional unit of nearly every song you could dance to. The problem is that writing loops out sequentially is very tedious.
Rather than writing the same passage over and over, you can save yourself a lot of laborious writing by using repeat markers. They’re like the GOTO instruction in BASIC. Here are the first four bars of “Chameleon” by Herbie Hancock. This four-bar phrase repeats hundreds of times over the course of the song. You wouldn’t want to write them all out. With repeat markers, you don’t have to. Repeat markers give sheet music the topology of a clock face.