- The heavy crossbow is the only "slow" personal weapon, firing every other round. In no-supplement OD&D, there's thus no benefit to getting a heavy crossbow; the +1 bonus is at least something.
- They're all machines, eliminating one variable in the use of the weapon. A +1 per die of damage is a tiny acknowledgment of the technological improvement.
Most other people creating house rules or mods prefer to increase damage to reflect better technology. This doesn't fit with my personal aesthetic, however; to me, the 1d6 damage roll represents how lucky a potentially killing blow is, compared to the average human warrior, just as the 1d6 hit die for a human warrior represents how lucky the warrior is against the average potentially lethal blow. The bonus to hit for catapults may not be that big a deal, although it helps o-level catapult crews be more useful. For hand-held crossbows, though, they represent the fact that a killing machine is going to be a little more likely to kill people than melee weapons, because it was deliberately made to be more lethal.
It's going to matter a lot more for games where I use guns. Because yeah, pistols and rifles are only going to do 1d6 damage (the horror!) What distinguishes them from other weapons is the increased likelihood of doing that damage, plus maybe a few other special characteristics, like piercing armor.