A lot of posters here would agree a deep farm yields both core players and prospect capital to trade for MLB areas of need; which, for the Red Sox rings, was usually pitching.
But free agents have a long history of putting teams over the top, whether you call them key or prime, or just Howie Kendrick, the 2019 World Series MVP.
Pre-free agency trivia (from an old Baseball Digest, circa 1968): the '67 Impossible Dream Red Sox won the AL pennant with 17 homegrown players on their 25-man roster. They actually did add a free agent that season -- Ken Harrelson -- signed after being released by the A's for bad-mouthing owner Charlie Finely. The Hawk was recruited down the stretch to replace injured slugger Tony Conigliaro, beaned out of the season in mid-August.
Unfortunately, Harrelson didn't approximate Tony C's All-Star bat in '67. But the Hawk led the bigs in RBI in '68 and finished third in AL MVP voting.