Why there? Mostly because Greenwich got there first, and did it better than others.
Let's ovesimply the story. Figuring out the time wasn't particularly easy to do on land, but it was much harder when done on a boat in the oceans. Scientists soon figured out that the location of the stars in the sky could be helpful. However, a reference point of longitude was needed. England's major observatory was in Greenwich, located on top of a hill overlooking the small town below. Workers took a look at a globe and decided the north-south line should go right through Greenwich. After all, it was their decision. However, other countries did the same thing, which was rather confusing.
But Greenwich was ahead of the world in technology, and many of its charts had become the standard. So when many nations gathered in 1884 to draw the lines formally, Greenwich was a natural choice to be selected. It's been set there every since. (Paris was a holdout for a few decades, but came around eventually.)
The fun part about a visit is that you can stand on both sides of the Meridian, and thus be in both the Eastern and Western Hemisphere at the same time. And tourists flock to the Observatory, mostly to do exactly that.
The Observatory has a rich history. Edmund Halley, he of Comet fame, worked there in the 1700s. So did William Herschel, who discovered Uranus in 1781. Part of an old telescope of his welcomes you at the entrance. There's some other historical items on display on the subjects of time and navigation.
But it's still a place to stand in the Eastern and Western Hemispheres at the same time, where the line was first drawn. That's rather impressive.
(Footnote: There's only one country in which you could be in four hemispheres at once: Kiribati in the Pacific Ocean. The other crossing is south and west of Africa in the ocean.)