With a regular access like what most of have i.e via trading terminal we get access to limited order book (top 5 bid and ask). While with a Direct Market Access (DMA) you get access to the entire order book, therefore allowing you to figure out the best liquidity pockets. DMA is best when you have a algorithmic trading set up in place.
Do you mean colocation with NSE server and having a broking license and becoming a direct member under NSE?