Installing Headers without Gaskets WTF ??

Get the Permatex Ultra Copper Gasket Maker here https://amzn.to/2XEOjk5 In Canada get the Ultra Copper Gasket maker here https://amzn.to/2XKfIB4 Installing headers without using gaskets ? that was something I have never heard of before. The instructions call for a using a high temperature gasket maker to seal the surface when installing the headers. This seemed odd but for the Ls Swap in my Jeep TJ this is what was required. Will headers without gaskets cause any issues will soon find out