The Milwaukee Brewers aren't out to knock the Chicago Cubs from the National League Central throne. They already did that in 2018. Their goal for 2019 is to keep the status quo.
This explains their interest in Madison Bumgarner. According to Jon Morosi of MLB.com, the San Francisco Giants have had "substantive communication" about the ace left-hander with the Brewers, who represent his "most likely destination" in a trade.
It's not guaranteed that the Giants will indeed move Bumgarner. There thus far hasn't been any indication that they hired general manager Farhan Zaidi away from the Los Angeles Dodgers just so he could carry out a rebuild.