Basically because the UK is the way everyone outside of the EU trades with the EU, because it's a pain in the ass to deal with their individual rules. Pretty much anything America or China trades to the EU is via the UK, which makes everything a lot smoother and cheaper for the US and China, gets money into the British economy and gets the EU their American and Chinese stuff. The pound is a pretty important world economic feature, being one of the strongest (often considered THE strongest) currency in the world.

Now it's devalued 30 years overnight, so anything trading in sterling has just lost 10% of its value and that's just the start.