8-20×50 binoculars Vs 20×50 binoculars? Olympus Vs Nikon Vs Bushnell?

I’m considering purchasing a binoculars. I generally see that binoculars with 8-20×50 are more costly than 20×50 binoculars. What is that the first binoculars can do that the second cant do?
Is it that with first binoculars I can adjust the magnification, where as with the second binoculars, the magnification is pre-adjusted?
Also can you please suggest [...]