I guess this is homework.
Think about the converse- what would happen if scientists weren't objective and tried to show what they wanted to show /hoped they'd find rather than the truth
Because their conclusions must be based only on the evidence they have collected, analysed and evaluated and not on bias, prejudice, wishful thinking, personal ego, or religious conviction.
scientist have to make sure there work is accurate so cannot be subjective. To give better representation to there study they must be aware of other confounding variables and make sure that they are not bias in there findings or getting what people call expeimenter bias (when the resercher finds what they are looking for)
It would not be a very good experiment or accurate if the scientist made judgments based on their own subjective views! :)