Grease on the threads is an old mechanics trick that oftentimes does the job. The grease itself isn't to improve the sealing. The threads might have a burr or slight imperfection and when you tighten the lines, they may not tighten all the way due to the imperfection. The grease will lube the threads and allow the fitting to be tightened completely thus resulting in a tight seal with no leakage.
Doesn't always do the trick, but often it does, and it certainly is worth trying before going out and spending money on another set of lines!
Agreed,...it's also a must do on Stainless lines/fittings as stainless galls easily, plus greasing/lubing the threads also makes disassembly easier years down the road regardless of the alloys employed...
