You get both, on thicker materials (4mm) you can double your travel speed without increasing your amps, this means less heat input and reduced distortion.

You find you get a lot less oxide discolouration on the weldmetal and adjacent plate due to the reducing action of the hydrogen and lower heat input, it also works well for root purging although nitrogen with 10% hydrogen comes out even cleaner

Cheers - Bob

The addition of Hydrogen of 2% up to 5% in manual tig on 300 series stainless steel has the advantage of increasing heat input into the puddle, resulting in a quicker stabilization of the puddle, faster travel speed, reduced oxidation of the weld, and because of the faster travel speed, a smaller heat affected zone. the issues with welding stainless, is the puddle is quite sluggish. additions of H2 increase the fluidity of the puddle resulting in beautiful looking, well wetted, gold hued beads.
I have done some testing for a customer with 2% and 5% H2 in Argon on some coupons. The weld was an outside corner with no filler rod, 3/16" material (316 stainless). The test was to show the increase in travel speed. test went something like this: when arc is initiated, stopwatch starts, fuse the two plates with 20 seconds of weld time. On the pure argon test, the weld length was about 4 inches. On the 2% H2 test, the length was about 7 inches. On the 5% H2, I ran out of plate. (test plates were about 12" long) The backside of the welds showed a huge difference in the heat affected zone with the argon bluing being about 1/2 to 3/4". the 5% H2 zone was about 1/8 to 3/16".
pretty interesting results.