i am examining this question for a chemical engineer's point of view:
steady state means there is no accumulation of intermediate species throughout the process. it's a valid assumption because:
1. there are no intermediates at the end of the reaction.
2. the extent of reaction between the product and intermediate is either zero or insignificant
3. the elementary step to form the final product is irreversible
it's a useful approximation because it simplifies the derivation of the rate law.