Lighting and reflections are incredibly important to make a scene look real. If either is slightly off, it can through the entire scene off. While Blender does most of the calculations in terms of light bounces and reflections, there are some things you can use to make your lighting and reflections better. Matching your roughness value to the material your working with is very important to this. For example, you don't want a shiny piece of metal to have a high roughness value and no reflections. As an addition to this, for even more realism the creation of roughness maps helps your objects to look imperfect and therefore more real. Roughness maps use textures to scale the roughness values all over your objects so that they are not uniform in their roughness. To improve lighting, use 3 point light setups and/or HDRI's to give accurate lighting. When using a 3 point setup, using a blackbody node to get different temperature lights helps your lights mimic real-world lights.