In addition to being the polite thing to do, sending the human player when the robot isn't available also ensures that someone on the drive team has had a chance to see how queuing is working.
If you are having a rough inspection day and don't get the robot out for any practice matches, it's nice to have at least one member of the drive team who's been through queuing before you report to queuing for your first qualification match.
This is particularly important if you are a rookie team, have an inexperienced drive team, or are at a regional you haven't competed in before.
Although the queuing experience is fairly standard and the queuing volunteers will take care of you, reporting for the first live match is much less stressful when someone knows little things like at what point the flag is issued (if there are flags), what the traffic flow is, where the cart goes when the robot is on the field, or any other venue or seasonal quirks. If you've had enough trouble that you're in that situation, anything you can do to bring the stress down a notch is worth the time.