Darklords are plot devices as well as NPCs. While they don't need to be at the center of every adventure, or even most adventures, they're a tool that DMs can use to initiate plots (e.g. what's Azalin up to
this time? okay, here's the teensy peripheral part of his scheme which the PCs run up against....), justify circumstances (e.g. why is all of Sithicus falling to pieces? no, my PCs won't be facing Soth, they're not powerful enough yet ... but they can certainly try to save this particular elven village!), and even to correct egregious gaffs on the DM's part (e.g. oops, I let my PCs set fire to the entire Village of Barovia! wait, here comes Count Strahd's magical fog ... it's sweeping over the village, better run for it PCs ... gosh, look at that, the people are okay and the fire's gone out! guess you guys got lucky that time, but I wouldn't try that again, not if the local wizard-lord's got
that kind of spell-power....). They can be the mastermind behind the curtain, manipulating the PCs' direct opponents toward some greater end the players don't suspect, or the
diablos ex machina that steps in to undermine a lesser villain's scheming at the last minute, if the PCs themselves fail ... but at a price far more terrible than that lesser villain's total victory would have cost.
Darklords also embody the theme or "flavor" of their domain, so provide essential inspiration for how a DM should portray a particular country's landscape, social climate, and/or cultural elements. For instance, both Necropolis and Keening are lifeless domains and home to undead that re-enact the activities of their past lives, yet the former is actively malignant first and tragic second, while the latter is the other way around ... just like their darklords. Descriptions of encounters in these two domains should
feel very different, even if they're identical in terms of game-stats, and skimming their respective darklords' descriptive text should help a DM find the right words to express those differences (e.g. "As it charges, the zombie's eyes flare with its hatred and envy for the loathed living..." vs. "The undeath-stricken soldier's open hand seems to reach towards you in a voiceless plea for release, even as its sword arm lifts its blade to strike...").
"Who [u]cares[/u] what the Dark Powers are? They're [i]bastards![/i] That's all I need to know of them." -- Crow