It's just to protect the smaller components from the heat.
To be frank, I just removed the motherboard and carefully apply heat as I disassemble the LCD/Digitizer.
Once I get an opening I just apply small amount of heat and go through the adhesive in the back. I've never had an issue with this process.
The only time I completely disassemble the phone is when I need to preserve the LCD, this is for S7 and lower that requires you to get some access to the LCD to remove the motherboard.
good luck