The Catalans moved within one point of leaders Atletico Madrid with Monday's 1-0 win over Real Valladolid, with third-placed Real Madrid a further two points back.
Atletico have dropped points in five of their past eight league fixtures, including Sunday's 1-0 loss to Sevilla, opening the door in the title race.
Barcelona make the trip to the capital to play Madrid on Saturday, while Atletico meet fifth-placed Real Betis.
"I think the Clasico is always important," Koeman said.
"Of course, it's maybe more important now because of the situation in the league, where we and Real Madrid are fighting against Atletico Madrid to win the title this season.
"Of course, it's an important game with two big, big teams in the world and of course it's an important result for us to get more possibilities to win the title."
Barcelona will enter the El Clasico in excellent league form, having won their past six LaLiga matches and going unbeaten across 19 matches.
Madrid are unbeaten in their past nine in the league and have won their past three LaLiga matches.
"It was really important to win tonight after the results in the league, to be more close to Atletico at one point now," Koeman added. "But also Madrid is close to them and will be really emotional until the end of the season.
"The team is working good. We're confident we can win the title but as we saw today, every game is difficult."