“Offically” in the manual, the Global Guitar PAD is for “hot” pickups.
The Helix and LT ship with it OFF by default – it measures in at approx. 5.5db in the analog pre-converter stage.
Like all real [ tube ] amp front ends, you need to give the pre-amp room to work and breathe – hit it too hard and it all starts to sound too-gained-up / too-spitty / too-fitzy.
If your guitar, such as a Les Paul Standard, has either mild-HB’s or P90’s… your tones will be much better with the Guitar PAD turned ON for all of your sounds.
If, with it ON, you need a bit more gain / grunt, just bump up the Amp Block gain.
With the Global Guitar PAD OFF, (while using a guitar with “hot” pickups) the tones are hot / gainy / compressed. This can sound “instantly gratifying” at home levels but once you turn the volume up at a live-gig, your tones will diminish in quality.
Keeping it ON globally can help in massively opening up the “air” in the gain and the EQ.