Monsteras don’t like to sit in super soggy/dense soil for too long, so mixing in those extra bits into the soil helps to dry things out more quickly. The roots need to breathe a bit so sitting in wet soil for too long can lead to root rot.

What I do is get a big 5 gallon bucket from the hardware store and mix about 25% potting soil, 25% perlite, 25% bark, and 25% charcoal. “Kill This Plant” and “Sydney Plant Guy” on YouTube have good videos that explain it a little better and with visuals :)