Code shares exist for only one reason. To divert unwitting passengers to interline connections that are in the interest of the airlines and not necessarily in the passenger's best interest.
In the good old days when airlines themselves owned the distributions systems the US DOT ruled that certain rules had to be followed when displaying/selling on systems uses by third parities (i.e. travel agents and other airlines). Displays/bookings done in house didn't have to follow these rules.
Flight displays had to be listed in the following order;
Non-stop flights,
Direct flights (flights with one or more enroute stops)
On-line connecting flights
Interline connecting flights.
There were algorithms for departure time requested and journey durations etc. The bottom line was that an on-line connection, even though worse for a passenger, would trump an interline connection.
The airlines effectively gamed the rules by showing direct flights, that were actually connecting flights, and by showing interline connections that were made to appear to be on-line connections.
All of this to the benefit of the airline and not the customer.
I remember a classic example of a journey from San Diego to Frankfurt on Northwest airlines. The journey actually involved a NW airlink flight SAN-LAX, a NW flight LAX-MSP, a KL flight MSP AMS and a KL flight AMS FRA.
the first lines of the display were the 'direct flights'
SAN FRA NW123 3 stops,
SAN FRA KL1123 3 stops,
Then the online connections:
SAN LAX NW456
LAX AMS NW123 2 stops
SAN LAX KL1456
LAX AMS KL1123 2 stops
SAN MSP NW456 1 stop
MSP FRA NW123 1 stop
SAN MSP KL1456 1 stop
MSP FRA KL 1123 1 stop
etc, etc, etc.
The best choice for the passenger, on the day, was a UA regional flight connecting to the LH non-stop flight to FRA. However this connection was pushed onto the second screen by all the devious fake direct flights and fake on-line connections.
Being on the second screen meant it was rarely sold by the 'typical' travel agent.
Since the airlines have now divested their controlling interests in the now-called GDSs the DOT decided to do away with these rules. Most, if not all, other regulators have done the same. However the GDS have shown no inclination of coming up with a better solution (that is better for the passenger) so the old rules are still being applied. Why not? If their customer (the airline) is happy they are happy.