For example, you’re cleaning a room, your home or your apartment. And you find it defeating the purpose to dust things and wipe them down because, dust comes back anyways and eventually. It might not be as bad as it was or as bad as it could be, but you’ll always end up with dust and cleaning dust is just a grand waste of effort and time.
Defeating the purpose would be more like washing your disposable cutlery that you’re just about to throw out. Cleaning the house is important.