NextGen - What is the difference between charge now and charge later for boarding?

NextGen - What is the difference between charge now and charge later for boarding?

CHARGE NOW: With this option, you can charge the customer as soon as the reservation has been created for a certain pet. The invoice will already be paid. 

CHARGE LATER: This option allows the pet to be reserved and admitted first. The payment can be collected from the customer after the pet is discharged from the boarding facility.