And for the sake of completeness, it is not just for man pages either. vim simply calls 'keywordprg' with the word under the cursor, so you can use whatever you feel like. Open a browser tab on your favourite documentation, display an ERD, pop open a picture of a kitten, …
Many filetype plugins come with pre-configured 'keywordprg' settings. Including some non-programming filetypes like git which will execute `git show <word>`, which is great if you're the type of person who often references commits in other commit messages.
I still regularly use this feature to look up libc and other things. You can also type 2K to go to ‘man 2’, or 3K to go to ‘man 3’ etc.