You always need roughness. Specular for non metals are typically uniform, very low intensity and non colored.(in range of 0.02-0.06) For this reason material system actual remap specular to range of 0-1 and default to 0.5. This material specular value is just multiplied with 0.08 to get physical based specular values. Most common materials does not need anything to plugged to specular slot. Some materials need constant value to be plugged to specular.(0.255 for water). Sometimes you want to bake micro occlusion texture and you need to multiply base color and specular with this texture.
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