One word: Documentation.
This one I keep comming back to. To give an indirect example: today I was was trying to remember the opposite of 'chr' function in Python. help(chr) only described the function. I had to Google and go to
https://docs.python.org/2/library/functions.html#chr to find out the answer. Why couldn't that info be put into the built-in help?
On OpenBSD by contrast, I always find myself reading man pages rather than Googling. It is so much a nicer feeling knowing that I have all the info I need in the man pages.