This advice comes from already-mentioned MFA vs NYC book, more specifically from the interview with the Paris Review editor Lorin Stein (adapted from the longer piece, previously published in The Days of Yore):
“And I would say, for people who want to go into book publishing or any kind of publishing, it is good to have experience writing really short stuff for really conventional magazines. Because once you learn how to spit out boilerplate, whether it’s for Elle Magazine, or Publishers Weekly, or Bon Appetit, whatever, that skill translates into all kinds of memo writing and copy writing and back of the book stuff and letters that you need to write. I think training in the world of schlock journalism can really give you skills that are useful in other areas of life.”
In fact, anyone interested in a publishing career should read the whole interview.