A simplistic view of the MSSP would be starting with the larger 28 pin devices, or possibly 20 pin (don't know), you get the addition of a master I2C. So with that, you get to read and write to other I2C devices and Pic Slaves (SSP) using built in hardware. The hardware master I2C most likely gives you faster, leaner code execution over a software version required on the smaller pin devices. The most obvious reason I can think of, is you get to get to read or write a byte at a time thru the SSPBUF register, instead of bit-banging in software. Someone else might have a better more detailed explanation.