A few reasons to do audition electives:
First, you expose yourself to the program. If you are a competative applicant, but not top notched, and you do well on your elective you can greatly increase your chances of getting moved up on the rank list. Programs would rather take people they know and work well with over a paper candidate who did a good acting job during the interview. Now, this does not always happen, for example, no matter how well you do on your Derm audition, if you have a 187 and C- average, you liekly aren't getting the spot. Then some programs always like to take AOAs and top of the class people.
Secondly, you can get a LOR from the elective, and if it's from some good name place with a well known Chairman, it will help you.
Of course you can bomb the elective, it's a double edged sword. Just remember to work hard, read, show interest, and play nice with everyone.