Minecraft wiki entry for Time-saving_tips, associated text: TNT is useful for rapid clearing of areas, especially when resource collection is not a concern. Two blocks of TNT can easily remove 50 blocks of dirt or gravel, and multiple blocks can clear entire stone caverns much more quickly than could be done by hand.
One block of TNT sets off other blocks of TNT when it explodes. Because of this, you do not have to set off each TNT, saving time. This doesn't work if the TNT is too far away from other TNT.