What happens when I rate a Steam Review funny?
Solution 1:
Saying that a review is funny does not do anything besides increase the counter saying "people found this review funny".
Solution 2:
Just to expand on the (correct) accepted answer, the 'funny' button was added to try to curb the number of joke reviews that would get upvoted to the top of the list despite not actually being helpful. This gives you a way to give the reviewer some kudos without necessarily saying the review is helpful in figuring out if someone would like the game or not (though you can also mark reviews both as helpful and funny).