double-decker is for higher capacity in comfort, comparable to hub service flights. It'll have few if any stops.
Articulated is for higher capacity in efficiency. People can get on and off faster, partly by having more doors, partly because they do not need to get up and down stairs.
The first image seems to show a double-decker in route service (Omnibus), which is a first for me. but it makes sense if you don't have any bridges to worry about, because the problem with articulated busses are they are tougher to drive. In many countries it require a special/plus license to drive articulated busses.>>1569448
I am a little surprised it exist, because it seems more appropriable to have two double-decker running a service i suspect that one might. But if there's demand for the capacity, and it's deemed safe to operate the given route with a single driver, i guess it's optimal right there.
But then you run into the problem of flexibility, sure a specific case can be found for that double-decker-articulated bus, but if that route suddenly isn't available/profitable/serviceable, what the damn hell are you going to do with that unit then? It's liability in a business sense, unless your dead safe in being able to utilize it, whilst being the extra maintenance worth it.