score:7
Put quite simply it allows an airline to offer more destinations to its customers.
It is marketing bragging rights, saying JR Air flies to 1200 destinations, rather than JR Air flies to 500 destinations but we can book you with our buddies to another 700.
JR Air frequent flyers can get standardized points all the way to their destination without trying to figure out cross airline point schemes.
JR Air can set fares between city pairs, as they have contracted for seats at a set fare(s), rather than looking at prices that are available when you book.
JR Air can instantly confirm seats because they have contracted for XX seats on each code share flight and sell from their contracted allotment rather than querying what is currently available each time.