Effectively, every win gains a game on half the league so a net half game gain on the league, and every loss loses a game on half the league so a net loss of half a game on the league.
To be in the wild card conversation, they'll need to gain 4-5 games on the league as things stand, so the Cubs will have to play 8-10 games over .500 for a stretch to be in the playoff hunt.
Alternatively, for the division, they just have to outplay 4 teams overall, so in a weak division, the potential is there to contend without being quite as good. In that sense, they need to gain on the Reds and Brewers now, so with them playing each other, the Cubs only gain by winning their own game.