It was an erstwhile junction, where the line bound to Dhanushkodi used to branch away from the Rameshwaram line. Now this line is currently abandoned ( currently there is a proposal to rebuild it ). Yet IR stuck to the old convention and follows it a Pamban Junction.
Similar case is applicable to Coimbatore junction too,. Coimbatore was having a layout similar to Chennai Central, where all the train used to have a rake reversal, Erstwhile there was a line which bypasses Podanur and merges with the line bound to Irugur. Once Coimbatore to Irugur new line was constructed via Coimbatore North , the Podanur bypass line was dismantled around 1956 yet IR adhered to CBE junction for more than 70+...
more... years. The same continues even now.