Probably the most important reasons for franchise partners - is to get the long haul traffic.
You live in Durban - on the coast 450 miles from Johannesburg - there are no direct flights to London. You can use one carrier to JNB and then collect your bags and recheck for the international sector. But South African Airways will offer you a through ticket. To complete like-for-like, BA need a partner.
Within South Africa PAX do not call it 'British Airways' they call it 'Comair'. That is the name of the company that has existed for many years and became a franchise partner. The planes have the BA logo in large letters but they are almost universally known as Comair because that is the actual name of the ZA registered company.
In America, they do not allow franchise, so airlines build up partnerships and codeshares. They do whatever it takes to secure the long haul seat. "You want to go to Europe? Why not use BA as your single carrier? From your local/regional field to London and then onward to wherever you want to go. Blah Blah."