Why Database use Buffer pool for write operation? In this case, first we write data in RAM,then same data write in database?

Why we need  do write operation two time?
If you are using SQL Server, write operations pull pages into memory if they need to be modified and write the changes to the page in memory as well as the transaction log. At given intervals those changes, and other changes made to adjacent pages since the last disk write operation occurred, are written to disk. This allows the server to make more logical writes to the disk in one go, making that write operations much more efficient.

Here are some links with good explanations (again assuming you are using SQL Server):


When things are written to disk: http://technet.microsoft.com/en-us/library/ms189573%28v=sql.105%29.aspx
What happens during a write operation: http://technet.microsoft.com/en-us/library/aa337560%28v=sql.105%29.aspx