To kill fleas, we obviously need to know where they are. That’s the difficulty with household sprays. Flea eggs will be dropped around the house wherever the cat or dog goes. Problem is, you won’t always know where that is.
 

There is only one place we know with absolute certainty that fleas MUST spend part of their lives, and that's on the cat or dog.
 

That’s why it is always better to treat the pet, not the house.