I have found that life drawing has made the most significant difference to my development as a figurative artist.
It’s a rigorous discipline to draw accurately, mistakes are obvious in the human form. (Incidentally mistakes are obvious in portrait classes too but the corrective action is perhaps more subtle and it’s difficult for a large group to get close enough compared to a life study.)
Life classes are also a fantastic opportunity to experiment with materials and methods. From painting loosely in turps washes to a pastel drawing made without my specs.
If you’ve only access to an untutored life class then it’s easy to get stuck in a rut and work in the same approach every week. I can highly recommend working your way through the Nicoliades classic “The Natural Way To Draw” to provide a framework to progress through.
Also see my post about double life class paintings. The double paintings have the additional challenge of identifying the relative skin tones between the models.