Basic render problem in Blender 2.8

A very NOOB question, I’m sure…
I assigned a “Glossy BDSF” to the default cube and this seems to affect the cube in the viewport of the shading tab, but when I go to the render tab and try to render this with Eevee or Cycles only a solid diffuse grey cube without any glossiness is the result. I tried the “Principled BDSF” as well and played with its settings (like “specular”), but this also only results in a solid grey cube without any specularity, glossiness, reflections whatsoever…
What am I obviously doing wrong?

A glossy shader just reflects the environment of the scene.
If world is just defined as a grey color without any variation, reflection that you should observe is just grey.

You just see what is correct according to environment in place.
If you turn around the cube, at a moment, you should see a white disk on top face as reflection of lamp.
If you enable nodes for world and add a texture to its color setting : you should see the texture reflected on cube.
To reflect another object of scene on cube with EEVEE, you will need to enable Screen Space Reflections or to add a Reflection Light Probe.

As @zeauro points out, to see reflections and bounce light etc in Eevee you need to enable other options, in cycles this will render in the viewport, just not in ‘real-time’ . . .

Basic_render.blend (657.1 KB)

Be sure to set the Roughness of the material closer to zero if you want a more reflective surface.

Ah, okay that makes sense… Thank you both!
But why does the viewport in the shading tab show an “instant” different look if that is not visible in the render?

LookDev mode purpose is to test quickly shading under different lighting set-ups.
In that mode, lighting is produced by HDRI textures that are different of World like matcaps are different from rendered materials.

Aha, thanks…