The filibuster serves at least two purposes that immediately come to mind.
First, it protects each party from their extreme wings. The party in power can blame the other party for not enacting extreme agendas. Most people are center-right or -left, not at the extremes, and enactment of an extreme agenda would likely cause a backlash against the party in power, when either the difficulty adapting (see below) or the flaws in the extreme agenda rear their head.
Second, it protects the country from lurching from extreme right to extreme left every 4-8 years or so. This promotes stability in the law, which is good for the people and businesses who have to arrange their practices in accordance with it. Also, if nothing else, centrism reduces compliance costs, and allows businesses to focus on their raision d’etre — making profits for their shareholders — and not paying asshole lawyers to explain to them about the new regulations they have to learn to comply with.