It's an easily workable metal, and apparently the easiest way to seal the hole made when they empty the vacuum chamber and / or produce certain welds.

https://www.wired.com/story/stanley-cup-lead-soldering/

They use lead pipes in the UK. Why I don't know, but I guess it's got some properties that make it ideal for keeping water where it's supposed to be.

FWIW it has been illegal to use lead piping in the UK for at least 50 years now. Of course it's still in older properties.