It is not totally unrelated to web development. The app sitting on the server can very well take advantage of functional features like closures, higher-order functions, immutability, referential transparency... for instance, you sure have collections that you need to transform or manipulate in whatever way. Functional programming helps here, and it is for a reason that its idioms are penetrating mainstream languages.
Functional features help in conciseness, testability, parallelization, and they can also provide native solutions to problems you would otherwise solve with patterns.
Update: there are web frameworks for functional languages too. Weblocks for Common Lisp, Lift for Scala. These are the ones I've heard of, there might be more... however you don't necessarily have to be purely functional -- for example Scala is not pure and should work with any Java framework, you'd still be able to use functional programming for the business layer, etc.
