I think your question is: Why is it useful to know the background, preferences, and wishes of an individual in the past before meeting their needs now?
If that is the question, the psychological answer is that all that went on before in an individual's earlier life is stored in the brain in long-term memory. Short term memory is only seconds long and can only hold 8 bytes of information, which is why elderly adults or those with dementia, alzheimer's disease, or some other neurological degenerative brain disorder forget what they just ate, but remember what color dress they wore to a party 40 years ago.
If you understand their history and preferences via a well cataloged intake process and gain information from as many sources as possible, then you can provide care in a form that is recognizable and comforting. You have to remember that they are not always in the "here and now" just because care givers are. They probably are somewhere in the "past/before now". Try to meet them in a comfort zone that doesn't stress them more than they already are. Allow individual dignity to the degree that you can.