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.
This isnt an example of “defeating the purpose”. This is something that might be considered pointless, like making your bed when you’ll just sleep in it again later.
Defeating the purpose would be writing the password to your computer on a sticky note and putting it where anyone can see it.
Why does everyone relate making the bed to dusting the place? Those are two completely different things. Making the bed serves nothing other than tidiness, whereas dusting is necessary to keep a clean house and keep allergies at bay.