Among pines, firs, redwoods and most excurrent trees (with central leader trunks), stubs or stumps of limbs that were shed are common and more apparent than they are among decurrent trees (which branch into many main limbs). The older lower stubs slowly but eventually decay and fall away as the trunks compartmentalize (heal over) where they were formerly attached.
However, wild trees are rarely completely without such stubs. As the older lower stubs are shed, newer stubs develop higher up. The worst of their stubs get pruned away only when more refined landscapes are developed around such trees, and they get pruned accordingly. If the trees get groomed regularly every few years or so, not many new stubs get a chance to develop.
When pruning out viable limbs, they must be cut cleanly from the trunk or supporting limb, without stubs. Since they do not deteriorate slowly before falling away, the trunk or supporting limb has no time to start the process of compartmentalizing (healing) over where such limbs were attached. Cutting away cleanly eliminates as much obstruction to that process as possible.
Pruning necrotic stubs from trunks of excurrent trees is not quite so important because the trunks have a tendency to start the process of compartmentalization as such stubs are decaying, and can actually constrict and crush stubs if they do not fall away efficiently enough. Nonetheless, necrotic stubs get pruned out when trees are groomed, just because they are unappealing.
So, no matter what, stubs should not be left when pruning. It is not complicated. It is actually easier to control a saw when it is up against a tree trunk or main limb. Yet, many who do not know better, and many who really should, more often than not, leave trees looking like this fir tree.