There is "Pride in your ride". A nice shiny boat looks good and can make you feel good.

As far as gaining 1/10th of a knot – BS. Maybe in theory if you are already sailing it to its maximum.
If you are over steering, pinching, sheeting poorly or any number of the other 1000 things you can do wrong to go slow then those same things affect both the boat speed and the potential you might have gained by having a beautifully polished hull bottom.
Of those performance related driver techniques, maybe your are 50%, now you have 1/20th of a knot available form a whole lot of expense and effort.

You can gain 2 knots just from practice and it is a whole lot more fun than wet sanding.