Creating something which is useful in day to day life is a better idea, because
it shows that functional programming can be used to create actual things that
people use.
To me, being useful in day to day life isn't an issue, because I know
that's true. If others think that's a problem though, then I think
that's another argument to build a web app.
Not only would it be practical, but it definitely would have the
opportunity to use advanced fp techniques like parallelism,
memoization, continuations, etc.