Prelim programs are single year internships that tend to be of use for two reasons: (1) people who have gotten into PGY2 advanced programs and need to complete an intern year, and (2) people who didn't get what they wanted in terms of categorical programs and want to basically audition for a job or enter the match again with a year of employment under their belt.
Most advanced program folks would prefer transitional year or prelim medicine paths because they tend to be easier than surgery in terms of hours, but prelim surgery might make sense for folks bound toward quasi surgical paths such as optho or IR.
In general, prelim surgery for people without things lined up or not having been close to getting something categorical can be dead end paths.
Hope this clarifies.