I'm a little confused as to the question here (if indeed there is one). Is this about standard interworking of routes (e.g. a 23 then becomes a 37 round trip, before coming back as a 23)? Or is it about the odd journey tacked onto the start of a vehicle duty (e.g. the bus on the 23 board does the 0615 37 each morning)? Or is it about more complex or indeed seemingly arbitrary movement between routes?
Growing up in the North, it was always more apparent that you'd have major routes where, by definition, there was more than enough going on to bother about swapping vehicles etc. However, in more rural areas, you would see a lot more complex interworking of vehicles, none more so than at United's Ripon depot. Aside from the single bus working on the 36 to Leeds, vehicles would flit from route to route so some elderly LH would do a Harrogate Express, then arrive back to do a couple of city services, before then heading out to York or somewhere.
I guess the other major issue is the challenge of congestion so that when the dreaded temporary traffic lights appear, it becomes expedient to have buses swapping between routes if only to maintain some sort of reliability.