Hi kemel,
please share more context about the problem you are trying to solve.
common use case is to have a 3rd text, not input that will sum the 2 other inputs, when one of those changes in the form, without saving them in the db
another example is a calculated field that you might want to save in the db just before the save, or not save in the db but calculate just after the read. for instance currency exchange rate, tax etc
for that please refer to db hooks
good luck,
Shlomi