Why there are other planets in our solar system?

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Would life on Earth be possible if Earth was the only planet in the Solar system?

The answer is perhaps , we can only speculate and know too little about the evolution of the planetary system to give any firm answer.

The Moon, for example, (not a planet I know) stabilises the Earth spin axis, preventing it from flipping, which would otherwise happen (on long time scales) with drastic consequences for the Earth climate and hence any life forms.

Jupiter (and to a much lesser degree the other outer planets) plays a significant role in the dynamics of smaller bodies (asteroids and meteroids), including absorbing them. If, for example, the meteroid and asteroid impacts on Earth were more frequent, that too would have drastic consequences for the climate and life.