In the short story "The Devil and Tom Walker" what was the Devil's purpose?
This depends largely upon how we define "purpose".
If we mean to ask if the Devil had a plan or objective, then, no, not really. It's generally accepted that a part of the Devil's nature includes the temptation and corruption of humans and anything good, but "purpose" usually implies some greater or overarching scheme, rather than a short-term action, so saying that his "purpose" was to corrupt Tom and capture his soul is, perhaps, using a word with too much emphasis for the circumstances.
On the other hand, we could say that the Devil's purpose was a literary one, in that he was included in the story for a particular reason and that this reason, or purpose, was related to the overall moral message and impact of the story. In this perspective, the Devil's purpose was to lend a supernatural element to the story and illustrate the depths to which evil has permeated the lives of the average person.
Scratch also mentions that he "amuses himself" through various corruptions, so his purpose may simply be entertainment, but this could also be a feigned tone to mislead Tom as to the severity of their arrangement.