Pay off mortgage with HELOC to save interest

I've seen everyone refer to this process, but then the next minute they're talking about cash out refi with a conventional 30 yr mortgage on the rental. I can't for the life of me figure out why you would want to pay all that extra interest and reduce your bottom line so drastically over 30 years. Am I missing something? I've done several hours of deep dive investigation as well as making an Excel spreadsheet to do all the calculations. It just makes no sense to do a conventional mortgage anymore or again, am I missing something?