Buffer remembers the last data you read from the drive so if you happen to want that data again you will get it immediately without any seek and read delays.
Buffer will also remember the data you want to write so when you write something to the disk it is written to the buffer and the computer will be notified right away that the write is done instead of going through the slow process of really writing to the disk.
Nevertheless the seek time is much more important that the cache on disk. Most operating systems cache data in RAM anyway so the effect of the disk buffer is minimal.