Supposedly for administration and knowing the staff.
Trident 1 and 2 s were divided into 3 flights, each had a manager, deputy, secretary and an office. Trident 3s iirc had two flights; 4 and 5.
Cabin crew had 8 flights which were organised in a different way with a different combination of routes and aircraft types. Some flew mainly 1s and 2s others mainly 1s and 3s.
It was deemed by BEA management that it was too difficult to fly more than 2 different marks of Trident not that there was much difference! I flew 4 types of DC9 in the 70s including one with a totally different fuel system.
Whilst the idea was to know and help the pilots it was more of a disciplinary set up IMHO and jobs for the boys.
One of my mates applied for the weekend off to get married but when the roster came out someone in the office had ordered that he should work so he swopped off the Saturday but unable to find a swop for the Sunday had to come in for an early morning flight.
rgds Alan