Hm? Understanding is important. It opens up possibilities for change and/or improvements.

You might call someone who deny evolution a thickhead or a zealot, ignore, and that's the end of it. Or you can try to ask "why do you think so", and turns out they only heard about evolution from their pastor, who spoke about some nonsense like "monkeys gave birth to human babies". From there, it might be possible to point that out in a way that would make them read a wiki article about it, for example. That is, make one real step towards changing the opinion.

Assuming the other side would care to answer the "why", however...