When searching for something it is often useful to have a baseline to reference things against. That baseline may not be accurate or all encompassing but it helps refine the search. In the case of Earth-like life, it helps make it easier to search.
Technically, life adapted to fit Earth’s conditions, so in theory maybe life could adapt to suit any planet’s conditions. With that logic, we have a case for scouring every planet to search for life. It would be a huge undertaking to do that for just the seven other local planets, and it may be that life can’t exist on those worlds. If we instead say we know life can exist in Earth-like conditions it narrows down the search area but makes the chance of us finding something much higher.
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