Manual:Maximum Transmission Unit on RouterBoards
Originally MTU was introduced because of the high error rates and low speed of communications. Fragmentation of the data stream gives ability to correct corruption errors only by resending corrupted fragment, not the whole stream. Also on low speed connections such as modems it can take too much time to send a big fragment, so in this case communication is possible only with smaller fragments.
But in present days we have much lower error rates and higher speed of communication, this opens a possibility to increase the value of MTU. By increasing value of MTU we will result in less protocol overhead and reduce CPU utilization (mostly due to interrupt reduction)