Application of buffer chip in serial communication
Without looking at the details, you should be able to figure that out once you read this and then go get the datasheets.

THere are three reasons to buffer:

1) to do the voltage translation from one domain to another.
- is the msp430 running at a different voltage that what the USB is specified for?

2) The driver provides higher current drive for a higher load.
- often the pins are limited drive (current capability)

3) to protect the more expensive part which might be more sensitive to ESD etc.
- added for completeness