I never really understands the logic behind cleaner fluid / HD cleanse..

Isn't the paint as clean as it is after IPA wipedown after polishing ? How would another type of oil help the wax which has oil in it bond better?

Marketing gimmick?

 

Both IPA and HD cleanse contain chemical cleaners, HD Cleanse also contains light abrasives that help to clean the paint and provide a 'key' for the wax to anchor to

The idea behind using IPA after polishing / compounding is to ensure that there is nothing 'filling' ( i.e. wax, oils, Kaolin, etc) the scratches, and they are actually removed not disguised

 

Using paint cleansers after polishing is more than just cleaning the paint. As Jon mentioned, it's basically the missing link between bonding a carnauba to a synthetic polished paint. We've tried both with and without paint cleanser after polishing. When using a carnauba, you'll get better application, a warmer look, and longer protection if you apply a paint cleanser prior to waxing.