Our natural concept of time is linked to the rotation of the earth and we define the length of the day as the 24 hours it takes (on average) the earth to spin once on its axis.
As time pieces became more accurate and communication became global, there needed to be a point from which all other world times were based.
Since Great Britain was the world's foremost maritime power when the concept of latitude and longitude came to be, the starting point for designating longitude was the "prime meridian" which is zero degrees and runs through the Royal Greenwich Observatory, in Greenwich, England.