Within a network it is normal to consider the users as belonging to groups. These could for example, be based upon job function – finance, sales, manufacturing etc. or by building, Peel, Maxwell, Newton etc. When breaking users up into such groups, it is also normal to consider them as belonging to different IP sub-networks. Once you have introduced such an IP addressing scheme then in order for one group to communicate with another or to access some common service like a server, then you need to interconnect the different IP sub networks. This is where you need a router.
Why use routers within LANs?