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.
Your example is not defeating the purpose, it’s futility. Defeating the purpose would be to vacuum first and then dust after. An “undoing” must occur.