This confusing use of the word “purchase” showed up in a journal article:
"On the second dimension of analysis, the purchase of CMC [computer-mediated communication] on the creation and maintenance of a movement identity has been a widely debated topic …"
Turns out, there’s another definition for the noun form:
"a hold or position on something for applying power advantageously, or the advantage gained by such application."
Mercea, D. (2011). Digital prefigurative participation: The entwinement of online communication and offline participation in protest events. New Media & Society, 14(1), 153–169. doi:10.1177/1461444811429103