If I've just entered "0.21" then when the confirmation screen reads "21" it's not immediately obvious that it's wrong.
It's so much less obvious that I'm going to ask you to explain the difference to me because for the life of me I can't see it?
It happens when you put in ".21", not when you put in "0.21". If you omit the leading "0" it might drop the leading "." too.
If I've just entered "0.21" then when the confirmation screen reads "21" it's not immediately obvious that it's wrong.