Why do spacecraft exploring the planet Mars land on it not during opposition, but several months later

Why do spacecraft exploring the planet Mars land on it not during opposition, but several months later, near the eastern quadrature of the planet?

The device takes a considerable time to fly from Earth to Mars. The most profitable thing is to launch the apparatus so that during the opposition of Mars it is approximately halfway to it. Then he will reach the Red Planet near its eastern quadrature.

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