When I was doing my master studies we were introduced to, I think, Miranda. One of the firsts home-work assignments was to write an interpreter which I had to deliver with a friend.
I had an Atari, with a PC emulation chip, but the emulation wasn't very good, and didn't work on all programs. Anyway, it didn't accept the environment given. So I ended up coding about a two hundred lines out of my head at home.
Next day, we had to give in the assignment. As usual, I was late, so with a floppy with source code and about a minute left, I met with my friend. He said to forget about it, drop the assignment and go to class.
I logged in on a Sun workstation, uploaded the source code, and ran it. It worked flawlessly.
That's why functional programming matters.