Can any one tell me why multilink interfaces are used between serial interfaces?

Multilink PPP (MLP) balances load over interfaces. MLP splits packets and sends the fragments over parallel circuits. This way, MLP improves throughput and reduces latency between systems. MLP provides a method to split, recombine, and sequence datagrams across multiple logical data links. MLP allows packets to fragment, and the fragments to be sent at the same time over multiple point-to-point links to the same remote address.

Multilink is a virtual interface which is used to bundle multiple serial interfaces to increase the reliability of the connection as well as the throuput of the router. 

Hope this helps!!! Let me know if you need further information. I will be more than happy to share.

Thanks for answers guys