Why would I ever use save(commit=False) instead of just creating a form object from the ModelForm subclass and running is_valid() to validate both the form and model?
In other words, what is save(commit=False) for?
Can you provide hypothetical situations where this might be useful?
That's useful when you get most of your model data from a form, but you need to populate some null=False fields with non-form data.
Saving with commit=False gets you a model object, then you can add your extra data and save it.
This is a good example of that situation.
Here's the documentation on the save method. Note that if your form includes many-to-many fields, you'll also want to call form.save_m2m() after saving the model instance.