The camera doesn't just write the image to card - it has to process the image as well. This means:

- all the colour correction, sharpness, saturation etc. treatment that you have defined
- convert to JPG and/or convert to RAW with the parameters you have defined (12 bits/14 bits)
- write the JPG and/or the RAW file to card

My D300 can do up to 8 images/second. A RAW file is about 15 MB and a JPG about 2MB. That's quite a lot of data to process. I think that the internal buffer is used to hold data that is waiting to be treated by the image processor as well as data that is waiting to be written out