For the same reason we use 6 digit grid references rather than 4 digit grid references - the GR positions your location in a smaller area:
4 digit e.g. SK 54 16 1000 x 1000 m (1,000,000 sq metres)
6 digit e.g. SK 540 162 100 x 100 m (10,000 sq metres)
8 digit e.g. SK 5405 1623 10 x 10 m (100 sq metres)
10 digit e.g. SK 54058 16239 1 x 1 m (1 sq metre)
That said, with the current state of the art for consumer GPS devices we cannot get sufficient precision to locate ourselves to a single 1 x 1 m square and even to a single 10 x 10 m square is a challenge. The EU Galileo GNSS project however is aiming at sub-metric precision more akin to professional surveying GPS devices which use two signals from each satellite instead of one, so we may be able to do it on our phones in a few years.