My company writes and supports a lot of custom Haskell software for our parent, an ISP up in Alaska.
What we gain is a high level of confidence in our business logic rules due to strict typing, fewer overall tests, and by avoiding a few small gotchas uncrashable runtimes.
Not to say it doesn’t come without problems. Poor IDE integration (I personally don’t care), questionable documentation at times, and a steep rewrite your brain learning curve.