Meaning well is not the same as being useful, and most partners desperately want to be useful. The good news is that the role of a birth partner is largely learnable. Spend an hour on it together beforehand and they walk in with a job to do, rather than standing helpfully near the wall.
Comfort is the first: knowing your preferred positions, where to press for back pain, when to offer water, when to talk and when to be quiet. Advocacy is the second: holding your preferences in mind, asking the questions you cannot ask mid-surge, and making sure you are spoken to rather than over. Logistics is the third: the bag, the car, the snacks, the phone, the updates to family, so that your mind is free for the one job only you can do.
A prepared birth partner is worth more than almost anything you can pack.
It helps to talk, beforehand, about how they are feeling too. Birth can be daunting to witness, and a partner who knows that in advance tends to cope better in the room. Agree that they can step out for a breath if they need to, that they can ask staff questions, and that their steadiness genuinely helps you. Calm really is contagious, and it travels in both directions.