Stainless Steel needs either backing or argon shielding to the back side when the back side melts. Argon shielding usually gives the best quality but Copper or Aluminum backing can also work fine depending on engineering or code requirements
TIG Welding Stainless Steel tubing with full penetration requires a purge in order to prevent severe oxidation on the inside.
There are some applications where purging the inside is an absolute must...especially in food service applications.
The manifold in the video is not required to be done ...
In this video ... you will see Tips and Techniques used for TIG Welding a Stainless Steel 16ga Butt Joint with argon shielding on the penetration side. Fully penetrated stainless steel needs argon shielding on penetration side in order to maintain properties