IDE controller can use interrupts to signal when it is ready to transfer data. This makes possible to do other tasks while waiting drive to be ready. That is usually not useful in MS-DOS but using interrupts frees the bus for any DMA transfers. Polling mode is used when interrupts are disabled. Polling usually gives a little better access times since interrupt handling requires extra processing. There can be some compatibility issues with some old drives when polling is used with Block Mode transfers.