I think pre-cooking vegetables in BIR's is almost soley down to expediency in order for them to produce finished dishes in a short time frame.
To properly fry onions for example can take anything from 10-15 minutes, most BIR's will have turned out a whole dish in that time frame. So to speed it up when fried onions are required they pre-cook them or pre-boil them or whatever they do so they can cook them in the finished dish much faster.
Does pre-cooking vegetables add anything to the flavour of the finished dish? Probably, if they're being pre-cooked with spices, but no more so than if you were to fry whole spices in oil first before you add your onions to be fried.
Experiment with it yourself, pre-cook some onions before frying them off in your finished dish and let your taste buds decide what difference is being produced.