Resistors don't make switches; transistors (among other things) do, though resistors are used as a component part. Long story short: Essentially you don't always (or even most of the time) want as much current as possible; sometimes you want to limit your current and for that you need resistors. This can be used to split a current between two or more wires (the wire with the bigger resistance will get less current and vice versa), or it might just be to prevent a short circuit (too much current will fry or melt whatever you're passing it through because circuitry consumes power as heat), but the point is that the amount of current you want passing through a wire or circuit isn't always "all of it," so you have to use resistors.