I’m not very sure about the correct use of the bidet, but my grandmother would pour some powder in it and take feet baths in it.
I always understood it was intended mostly to clean the genitals/anus for people who had some health issue in this area. And also in the paste as a douche, right after an intercourse in order to limit the risk of pregnancy (I noticed the wikipedia article states it’s untrue, but I read a couple references in old novels to a woman rushing to the bidet after an intercourse).
I doubt it’s ever used right after going to the loo in france if only because the bidet is situated in the bathroom, while the toilets are normally situated in an independant room (so, you would have to dress up, leave the toilets, go to the bathroom, and then only use the bidet).