If you google “PBR Guide” you’ll find the handy two-part guide to PBR material put together by Allegorithmic. Specular is normally left at default unless it’s for a dielectric material whose specular level falls outside the normal range. Stuff like gemstones and the like.
It’s effect is fairly subtle in most cases.
EDIT: Tweaking Roughness will give you control over reflections. Turn it down for sharper, clearer reflections. Don’t forget to turn metallic up to 1.0 if it’s a shiny metal object. Adding a suitable texture as the input to Roughness can help a lot too - it get’s rid of the perfectly even reflection.