Assistance with defence would be a good starter. Knowing the UK and other Commonwealth nations would have your back if you were invaded would be a great reason to remain part of the Commonwealth.

Tourism is also a big factor when it comes to having the Queen as head of state in the UK. People want to visit and see the guards and palace buildings.

Most countries now in the Commonwealth are independent anyway, we just retain the Queen as head of state. I live in Australia and while the Queen very rarely is involved with our lives/politics, many of us are fine having her as the head of state. Why remove traditions that we've had for over 200 years?

They're free to leave anytime, but I don't personally see any plus sides to leaving the Commonwealth apart from your own full independency... Which could lead into the wrong hands.