A lot of artists will practice anatomy by drawing people nude, largely because it’s hard to get a good understanding of anatomy by only drawing people with clothes on.

If you wanted to put some examples of bare human anatomy in odd positions to expand the range that the model is capable of, well there aren’t many large corpuses of that than porn.

Also, even if they don’t want it to make explicit content, they probably want it to make “suggestive” or “appealing” content. And they just assume they can guide rail it away from making actual explicit content. Although that’s probably pretty short sighted given how weak guardrails really are.