The Monster Manual's Devil entry describes how devils desire souls and enter into contracts with mortals to gain them.
To own a creature's soul is to have absolute control over that creature, and most devils accept no other currency in exchange for the fiendish power and boons they can provide
However, it seems to make little difference what the creature used to be in life, a common powerless beggar, or a noble, high-level paladin. They all just turn into lemures:
When the soul of an evil mortal sinks into the Nine Hells, it takes on the physical form of a wretched lemure.
As this question asks tangentially (the main question is about ways to create magical pacts): why would a devil even bother to try and seduce an archmage or high level paladin, which is a lot of work as these victims can be resourceful, smart, wealthy, hard to seduce, and therefore costly and difficult to win. On the other hand, the devil could just offer modest riches to a beggar, who might be a lot more susceptible to to escape their wretched living conditions now and trade their soul for it. That seems like a lot less work and risk, and in the end, the devil gets a soul-turned-lemure either way.
Is there something inherent in a soul of a powerful, high level creature that makes it more valuable to devils and other traffickers in souls (night hags, for example), than a poor commoner's soul, and justifies the extra effort?