It gives players a better motive than just "catch em all" which most people can't do anyway without either having attend every single Pokemon give away event, trading (which some people may not be able to do: no internet for wifi=no online trade, perhaps too old to have a decently Pokemon fan-ed friend who is willing etc)whereas on mystery dungeon and ranger, not only does it give an insight into the lives of Pokemon themselves and the rangers, it lets us have all the means to get all the Pokemon, just have to have the whereabouts knowledge.