The reservation simply holds space. The ticket is what pays for the space. You don't technically pay for airfare with a credit card or cash. Instead you pay using a ticket that you bought using a CC or cash. If the reservation changes, you need a new ticket to pay for the new space.
By having them be separate you can also book more than one airline at same time. You can also get moved to OAL in IRROPS without needing to get a refund and then buying a flight on the new airline.