The purpose of this feature is to prevent runaway RAM usage when the computer can download faster than it can save pieces to disk.  This can especially be a problem when the downloaded files are being written to a network drive or a flash-based device.

You can adjust the parameters in Settings > Transfers > Files by clicking the Configure button next to "Piece creation limiter".

There is also a special diagnostic view of the total number of pieces waiting to write to disk.  It is available from Help > Diagnostics menu.

It is generally best to use the default settings unless you notice a backlog and want to adjust the RAM usage.