Life modeling is holding poses (usually between 1-30 minutes) in the presence of artists while partially or completely nude. It can be full figure, just portrait (neck up) or anything in-between.
Rendering an image of the human form is one of the oldest known art forms in all of human history. Ancient cave drawings would invariably contain drawings of humans in every day life. During the Renaissance, the great artistic masters would spend a great deal of time studying and rendering the human form. Before a great oil painting would be attempted, many sketches of the figures involved would be rendered before the final was even touched. Every skill essential to drawing from the figure will help an artist in any other creative endeavor. This is the reason that very art program in modern academia requires its students to draw/paint from the nude human figure. The skills are that basic and important.
Models sometimes pose with clothes on. These poses are called “draped.” Models are always for portrait, and occasionally for an effect or to intimate an emotion, the model dons a costume. Most often, the model is completely nude. This affords the artist the ability to see the muscular structure spread upon the skeletal structure, identifying landmark features, and understanding anatomical design and function. One can paint a landscape and get proportions wrong, but the human form must be rendered correctly. You simply have to understand how the body works in order to render it authentically.
For these reasons, in this one specific arena, under a very strict basic protocol, the normal societal structures against open nudity are suspended. The model disrobes, strikes a pose, and then holds perfectly still… whilst artists look upon him/her with great focus, and attempt to render a likeness.
I have a coin that I carry called a tetra drachum. It’s an ancient coin from the reign of Alexander the Great, struck more than 300 years BC. On one side is a portrait of Hercules wearing a lion skin cap, and on the obverse side is a full figure of Zeus with a staff and eagle. Even here, over 2300 years ago, there were artists who sculpted from the figure.
To pose for artists in the nude is an old craft, and an honorable one.