Eventually, some of your code will make a call to IO: reading from or writing to disk, responding to a web query, updating a DB, drawing something on the screen, etc, etc.

For example, you could put up a web server written entirely in Scheme or some other dialect. This would process and respond to queries entirely in Scheme, including some sort of engine for generating and serving html. Hacker News itself is an example of this: it's written in a lisp dialect called Arc.

Incidentally, Haskell is functional, but it isn't a lisp.