While that was the last version Lyman put out, I think earlier versions might have been more generous. That's why I prefer the chart
@BillinOregon posted, as opposed to the newer one. I strongly suspect most modern barrels can take far more than 120gr 2Fg or 3Fg without any additional wear, tear, or risk.
I am very skeptical of the numbers provided by most manufacturers. Many, like Pedersoli and most custom, semi-custom, and kit makers don't provide any charge data at all. The current Pedersoli manual simply says use the "proper charge" without any further elaboration. Tennessee Muzzleloading lists the same maximum (100gr [no size info]) for .50, .54, .58, and .62 calibers. Traditions lists the .54 caliber maximum as 125gr for 2Fg and 120gr for 3Fg.
Investarms, which made the Lyman GPR and now sells the same model under their own brand (the promo pictures are actually Lyman product photos), lists the maximum load as 100gr for 2Fg and 80gr for 3Fg, well below the Lyman maximums of 120gr 2Fg and 100gr 3Fg.
I have several .54 cal rifles. If I ever used over 120gr, I have no clear recollection of it.
You ask a good question though, "why any more powder is necessary"?
For target shooting or competition, I personally prioritize accuracy and economy. If 80gr is more accurate than 70gr, I'll go with 80gr. For most of my rifles I have two loads, one for hunting, and one for everything else. Hunting is where I think a case can be made for as much energy on target as possible.
In modern defensive handguns, I know, after accuracy, many favor volume of fire (capacity) and/or penetration. Based on what I've read, I favor very high velocity, high energy projectiles. This is all "matter of degree" differences and we can agree to disagree on this. With respect to hunting, I favor prioritizing a humane kill. Sure, one might take a deer with a 70gr load yielding only ~400 ft-lbs at 100 yards, but I don't favor, as an objective, playing around to seek the weakest load.
We're fortunate we're allowed to hunt with muzzleloaders. In Poland, muzzleloaders are not permitted for hunting, and neither is archery as far as I can tell. They have minimum energy requirements: 1000 joules (737 ft-lbs) at 100m for roe deer and 2000 joules (1475 ft-lbs) at 100m for red deer, etc. Only the .54 caliber 160gr over-charge meets the lower standard, roe deer, which are about half the size of white tail deer.
To put this in context, at 100 yards, a Winchester 30-30 (170gr Fed HS SP) is only generating 1109 ft-lbs, well below the higher requirement for red deer, (about half way between a mule deer and elk). A smokeless 44-40 fails to make the grade for either at 448 ft-lbs at 100 yards.
I really don't want such laws to come to the US. I prefer to use the most accurate, most powerful load possible to minimize the risk of a bad incident ending up on Tiktok and giving hunters a bad name.