The theory has to make strong statements about evidence. If the statements aren't strong, then the theory fits any evidence, and is unfalsifiable . It's bad for three very practical reasons. First, a theory which can't make predictions is a dead end. Second, it would be useless.
Oil companies are very pleased that geologists can predict where to drill for oil.
And third, if we have two rival theories, we want to use evidence to choose between them. If they are unfalsifiable, then evidence doesn't do that for us. The classical scientific example is Einstein's Theory of General Relativity. Einstein didn't just expect everyone to believe him. In his 1916 paper, he said that the Sun's gravity would bend light.