Removing the mirror and pentaprism makes the mirrorless camera smaller and cheaper. The digital viewfinder shows you exactly what the sensor is seeing, so you

Removing the mirror and pentaprism makes the mirrorless camera smaller and cheaper. The digital viewfinder shows you exactly what the sensor is seeing, so you have a better idea of what the final image will look like. They focus better during video. I think there are also some technical details on how the auto focus points work better mirrorless and the depth of field in the viewfinder is more accurate. I think DSLRs hold the apature wide open when you look through the view finder, then stop it down when you take the photo -- this keeps the view finder bright but effects the depth of field. Not 100% sure on this though. But a SLR or DSLR with a bright 100% view finder is a thing of beauty. So big and crisp and clear.