I can see that people think it might help with sleeping. Whether it does or not probably depends on the baby and the timing. But i can see how if you have a bad sleeper it would be tempting... Same as some BFers are tempted to try formula for last feed.
What I really can't see the point of is switching to formula at 6 months if you are BFing (unless you're going back to work or something). So you've done the hard bit, the constant feeding when tiny bit, you wait until they are having solids and the BFing has settled down and is getting less and takes less time and.... you stop and pay £££ for formula instead. Bonkers.