To add to Ocn Vw 1K's response, the idea is that your protection flight is therefore your worst case scenario to get to your destination. You're protected from having to live at the airport for days on end, waiting endlessly for a standby seat that with today's load factors might not materialize for a very long time.

In practice, I will add myself to the volunteer list at any time that the gate agent is willing to start one, regardless of whether the gate agent has already looked up possible protection flights and if so, whether the described protection flight is acceptable to me. Often as the time of departure gets closer, the protection flight offered by the gate agent gets better and better (higher cabin, fewer connections, arrival time becomes earlier, etc.) Getting onto the volunteer list just gives me the right of first refusal; until the gate agent tells me I'm needed, offers a specific protection flight, and I sign the form, I haven't lost anything. When I do volunteer, I consider the protection flight only the beginning point. Sometimes I'll look at my options and standby for earlier flights than the protection flight that the gate agent was able to find positive space to confirm me into. Sometimes I'll deliberately not standby for earlier flights in hopes of getting another bump on my protection flight.

Then again, that's exactly how I usually treat my original booked flight, as if it were my "protection" flight.