The problem I'm guessing they encountered was the need to try and balance the weapons against… let's call them "toughness" targets, versus heavy vehicles. Let's take poor old inaccurate, near-quality-immune overheating plasma for example. Poor plasma…
Now in the tabletop this weapon is generally known as your go-to versus "TEQ". In other words, Plasma penetrates Terminator armor. Wrecks the buggers quite well indeed. We could delude ourselves and pretend they'll never face down a terminator or greater daemon or a vehicle, but let's be honest here: FFG's 40k RPGs have more crossover of equipment and bestiaries than most.
1d10+7 Pen 6 cannot actually harm a TB 8 terminator without rolling a 10. The weapon best known for cutting down terminators can't so much as TICKLE one without being set to maximal, and then don't expect much. Thanks to Overheat interfering with the jamming numbers, quality and reliability, you'll run out of ammo or kill yourself before he drops, most likely.
The 1d10+8 Pen 10 human-version of BC is significantly better, punching through standard power-armor even in the chestplate. Semi-auto fire will take one down in probably two rounds, assuming his field fails to block the shots. Perhaps even one, on maximal, but then the risk is high.
At the other end of the spectrum, a Lascannon vaporises the guy on impact usually [even on the proven minimum of five 3s, you've probably dealt 13 wounds]. Still, it struggles against a Land Raider.
Still, we want variety in weapons right?
So we add penetration against vehicles. Can be nice and listed in the class [Basic 4] or [Heavy 6] and so on
Say, on average, Las has +4 but only gets it if Heavy, SP probably takes +4 as well but also applies to single-shot basics or whatever, Plasma and Melta get +6, but Meltas are getting doubled in Melta range so its okay [2d10+10E Pen 32 actually starts having a chance at doing damage to those AP 50s at some kind of rate], and Exotics vary greatly, but Pulse is probably just +4 as well.