French vs American Croissant Recipe

I noticed there are some difference in French and American croissant recipes. Mainly french recipes include allot of butter in the mixed dough (20% of flour weight), allot of instant yeast (+2%), and do not include milk. American recipes on the other hand will include less butter (10%), less instant yeast (+1%), and will include milk.

What is the reason for this difference? Put differently what is the benefit of including allot of butter or instead milk in the mixed dough for example?