Why there is kosher soap for Pesach?

Shalom, Thank you for your question – it is nice to be looking ahead to Pesach already! First let me start with the conclusion – soap for washing oneself with does not, according to the letter of the law, need any supervision for Passover, and all soaps may be used. The reason why some people prefer kosher for Pesach soap is based on an opinion that "anointment is akin to consuming" – that is that when one anoints the body with a soap or cream, it is as if one has drunk the soap. This would mean of course that one should be careful all year long to only use "kosher" soap, which was manufactured without animal fats etc. This is indeed the opinion of the Gra, (see the Biur Halacha (326:10 B’shaar)). However, even though most Jews are lenient throughout the year, there are those who like to be strict on Pesach, because of the stringency of chametz. (There are other issues involved, but this is the simple explanation). Even according to this stricture, the only problem would be soap with chametz in it. Again, let me stress that the overwhelming accepted practice is to use all soaps on Pesach, and not to require any supervision (see the O-U Passover lists). Blessings.