The big use of having lots of resistors (or sensors) is that we can use them to split up potential differences and channel them to a particular use.
http://techitoutuk.com/knowledge/electronics/buildingblocks/potentialdividers/
What you need to know is the formula for a potential divider and be able to apply it.
http://www.youtube.com/watch?v=Mn2i3DPI-a4
Questions and worked solutions can be found here: http://tap.iop.org/electricity/circuits/118/page_46038.html
Where this becomes really interesting is when we have a sensor (LDR, Thermistor) in the circuit but more of that later.
Year12 Physics 