Rahsin,
A Router's primary role is to connect a LAN network with the Internet or an ISP, while a multilayer's switch primary role is to switch packets within the same LAN.
I've also seen cases where routers have replaced multilayer switches within a network and in most cases, its not the best design approach. The most common reason we see routers being used as multi-layer switches, is because they are usually more cheaper to provide this functionality (interVLAN routing), as opposed to layer-3 switches which are considerably more expensive.
You can read up on InterVLAN-routing and various methods this can be supported on our popular article
InterVLAN Routing - Routing between VLAN Networks
.
One thing you should be aware of is that multilayer switches are much faster than routers as packets are switched at wire-speed, where as routers introduce a delay which cannot be avoided (due to the packet processing mechanism) and increases as traffic increases on the router's interfaces, ultimately creating a bottleneck.
Hope this helps!
Chris.
.
