I have a few friends and acquaintances in the transport community, and this same question I did ask all of them in one moment or another.
From a time at work/to work perspective, since their vehicles are limited by speed control, but they also have a driving time limit after which they need to forcefully stop and "rest", each mile journey is precious, counterbalanced by fuel consumption of course.
So, begin able to overtake and move ahead a good stretch before time to "rest" comes, is important. Sometimes these people are journeying for several hundred, when not thousand of miles. And rest intervals are dictated by time, not mileage.
Let also consider that this can mean to "make it or break it" at fixed points, ferries, borders, and the like. When a few min can make the difference to missing a ferry, or hitting the border control in "break" (why they need to break while at work is a mystery), or even get to a pleasant stop instead of a dreadful parking stop in the middle of nowhere.
Moreover, there are differences in the vehicles themselves, while they all look alike, mechanically some can be better than others, and drivers exploit these advantages. A friend was telling me when he replaced the tires on his tractor, he chose a slightly rolling diameters meant he could "trick" the tachograph by doing couple mph more than was the instrument would be recording, and this was an advantage that could enable to overtake more "colleagues".
And finally sometimes they get tired, and simply swap leads when in convoy, even if keeping same speed.
Now true this "bothers" other drivers, and several of these guys really don't give a d@mn, but on the other hand we "car" users are far worse on our behavior on the road. We often forget these behemoth need large spaces for braking and their handling does not allow for fast reactions like a car would. Is a matter if finesse and respect on the road, which seldom appears.
I just came back from 1500km on SE Europe highways, and I saw in proportion much more dis-respect from 4-wheelers than from 18-wheelers. Sometimes is a matter of simple dynamics of travel, let me give you two examples which I went thru, among many: I am on a long stretch of road doing the speed limit, I come upon a convoy of two 18-wheelers, the follower noticeably faster and edging closer to the lead, thus getting ready to overtake. This will not hinder my progression, as I am fairly far and my speed differential allows for this to take place safely, but what to do? This is where the complex combination of skill, experience, know-how of the road and vehicles, and respect come to play.
One time the follow truck waited for me, so I raise my speed a few kph to bridge the gap faster, pass, and then he started overtaking. I thanked and he replied in return.
On another occasion, I flashed (once!) to warn and dropped a few kph to increase the speed gap, to which he started the overtaking and I barely got closer enough "to be bothered" ... moves to the right and I keep on. He thanks me and I reply in return.
Obviously is not always that nice, there are horrible places where traffic is hellish, is a dog eat dog scenario, but then this is not really the fault of the road users, but those who plan, build, and maintain that infrastructure, and get paid for its use as well!
I have great respect and admiration for the drivers of LGV/HGV and other heavy haulers. They live an awful life in horrific conditions, bad food, poor rest, miserable work conditions, tight/impossible deadlines, decadent infrastructure, mean border patrols, a lot of very stupid road users, and all this to make sure we get on our tables, wardrobes, and houses and lot of stuff we surely don't really need, but want to have. It is a vicious circle, we need them to have our junk, so we have to deal with them on the road ...
It is a fascinating aspect of the "modern" mobility of mankind 🙂