Heat rises. That's an unavoidable fact. The heat put out from the gpu comes from the side next to the window. The fans suck air from below and pump it out sideways. So most of that heat has an almost direct pathway to the top of the case.
The cpu cooler exhaust will dominate the rear exhaust, fan vrs fan, so there'll be little room to remove a majority of the gpu exhaust, it'll slowly leak out from the top mostly. This is where the top-rear exhaust comes into play. The draw vacuum from that fan collects most of that heat and helps it out the top. So you have a chimney affect. Cooler air in from below, hotter gpu exhaust shoving air up the side, top fan sucking it out the top in a constant flow.
Without that fan, you'll be relying on the ability of heat to escape out the top naturally, which it will, but not as fast as if helped by the fan. The remaining heat will sit there and eventually collect enough to start affecting the cpu cooler efficiency. The and cpu temps go up. Slightly.