Dipping is used to reduce chance of parasites being transfered to display. It is a pre quarantine practice.
It is performed on sps and some lps. Sps are prone to rtn especially wild harvested pieces.
Aquacultured are less prone to rtn. Some lps like elegance are subject to infections and may respond to dipping.
Dipping preparations can be as simple as 10 drops of iodine in a liter of seawater to commercial preparations.
Supposedly SPSS aren't big fans of dipping but I have always dipped them without a problem. I have dipped and QT'd EVERY coral that I have gotten after my tank was infected with zoa eating nudis. It's too much of an investment at this point to lose it all.