Why do they sometimes have a flight with 1 flight number that has a stop instead of just making it 2 flight numbers?

Example CO used to have flight 66 LAX-CLE, then continued CLE-LGW.
FLight 67 was LGW-CLE then CLE-LAX. (flight 66 no longer exists since the switch to LHR from LGW)

You had to deplane LGW-CLE in CLE to clear customs then get another plane anyhow....

Just wondering why..and do you get 2 segment points credit if you go from LAX-CLE-LGW or just one because 1 flight number.

A current example is flight 62 IAH-EWR-MAD.....